CORAL REEF PROJECT-PAPERS IN MEMORY OF DR THOMAS F GOREAU 15

WEST INDIAN MOLLUSCAN COMMUNITIES IN THEROCKY INTERTIDAL ZONE A MORPHOLOGICAL

APPROACH

GEERAT J VERMEIJDepartment of Zoology University of Maryland College Park Maryland 20742

ABSTRACT

Morphological and biogeographic analyses were carried out on theintertidal rocky-shore gastropod molluscs at five localities in JamaicaCura9ao and Guadeloupe West Indies Gastropods living on opensurfaces at low shore levels in wave-exposed areas have rather uniformlyhigh expansion rates W large apical half-angles A2 large posteriorgravitational stability angles GT and large lateral gravitational stabilityangles Gw apertures tend to be ovate to round They may vary greatlyin size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore habitats may take on lower A2W Gl and Gw values than their more wave-exposed counterparts andthe aperture may become elongate Littoral-fringe species on opensurfaces may also have A2 W G1 and Gw values somewhat lowerthan those of wave-exposed lower-shore species but the aperture isuniformly ovate to round Species in this category range somewhat lesswidely in size than at lower shore levels High-shore splashpools containa limited fauna of small species with ovate to round apertures and widelyranging A2 W GT and Gw values Among lower-shore species livingcryptically in crevices or beneath stones there is a great diversity inall parameters except size which tends to be small Thus where sizediversity is greatest morphological diversity appears to be least andand vice versa The relative morphological uniformity on low-shorewave-exposed open surfaces may reflect morphological restrictions im-posed by the requirement of adapting to water turbulence Rigorsof temperature and desiccation impose certain restrictions on form inopen-surface species of the littoral fringe

Unique features of West Indian intertidal molluscan communitiesas compared to those in other parts of the tropics include a highlydiverse littoral-fringe fauna particularly of littorinids a rather high di-versity of open-surface keyhole limpets (genus Fissurella) the absenceor near absence of such typical intertidal groups as cowries (genusCypraea) pulmonate limpets (genus Siphonaria) conids cerithiids andopen-surface mitrids and a lack of substratum specialization among upper-shore neritids and limpets The most substantial differences betweenphysically similar intertidal communities in different parts of the tropicsoccur among open-surface assemblages at low shore levels and particularlyin the littoral fringe The geographic distribution of species in thesehabitats is significantly smaller than in more sheltered open-surface andcryptic habitats

[23(2)352 Bulletin of Marine Science

INTRODUCTION

It is a common observation that terrestrial COmmUnItIesunder similarphysical regimes but in different geographic regions support species whichthough taxonomically unrelated are morphologically convergent and oftendivide up that habitat in similar ways (Richards 1952 Cody 1968 1970Mooney amp Dunn 1970 Leigh 1971) The reasons for this morphologicaland ecological convergence are not often known in detail but in generalreflect the way in which a particular body plan or group of body plansinteracts with the external physicochemical environment While geographi-cally isolated marine communities differ less strikingly from one anotherin a taxonomic sense than do many terrestrial communities morphologicaland ecological convergence within and between communities is neverthe-less apparent (Stephenson 1948 Thorson 1957 Jackson et ai 1971)Close examination of geographically isolated convergent communities oftenhowever reveals subtle but important differences in morphological composi-tion and ecological structure which may reflect the effects of historicalaccident Thus it is well known that mainland and island communitiesoften differ from one another in species diversity number and types ofpredators and in other ways as a result of patterns of dispersal and im-migration (MacArthur amp Wilson 1967 Leigh 1971) Consequentlyisland biotas become biologically unique relative to those of neighboringmainland areas and species living on the island tend to restrict theirgeographic ranges while adapting to the unique biotic and physical condi-tions at hand (MacArthur amp Wilson 1967 Vermeij 1972)

Intertidal molluscan communities are ideally suited for a study ofmorphological and ecological convergence between geographically andtemporally isolated communities Not only is the intertidal zone a con-veniently accessible and widely distributed habitat with clearly definedboundaries but the molluscan shell is geometrically relatively simple andtherefore amenable to comparative mathematical analysis The presentpaper is the first in a series dealing with morphological analyses of intertidalmolluscan communities under different physical conditions in the samebiogeographic region By analyzing the relation between form and habitatin the several Recent marine biogeographic regions it is hoped to lay thefoundation for the interpretation of similarities and differences betweenthe communities in these areas which are subjected to more-or-Iess-com-parable physicochemical conditions

Most previous literature on intertidal communities has been concernedwith the description of patterns of vertical and horizontal plant and animaldistribution on the shore (for reviews see Southward 1958 Knox 1960Sacchi 1969) A smaller body of literature has dealt with trophic structureand competitive interactions (see Paine 1966 1969 Connell 1961a 1961b1970 Batzli 1969) Although morphological studies have been carried

1973] Vermeij West Indian Molluscan Communities 353

out on particular species or groups of species of intertidal molluscs (egsee Staiger 1957 James 1968 Struhsaker 1968 Giesel 1970) almostno information exists on general morphological patterns among speciesin different habitats within one biogeographic region Most attempts thathave been made along these lines (Taylor 1968 Coomans 1969) havebeen qualitative and incidental and no rigorous morphological analyseswere undertaken Similarly while some data are available on faunisticand floristic differences between biogeographic regions (eg Ekman 1953Knox 1960 1963 Woodring 1966 Macnae 1968) no quantitative in-formation is available concerning differences and similarities in morphologyin comparable habitats in different geographic areas Nicol (1964a 1964b1965 1966 1967 1968) has made some general observations on mor-phological features of bivalves in different parts of the world but has notapproached the subject from an ecological point of view Stanley (1970)has made a valuable contribution to our understanding of the relation be-tween bivalve shell form and habitat but biogeographic differences werenot emphasized by him Elsewhere (Vermeij in prep) I have given anaccount of morphological patterns in high intertidal and supratidal gas-tropod assemblages suggesting that many properties of the shells oflittorinids neritids and limpets are adaptations to the rigorous conditionsat high shore levels

In the present paper the molluscan communities at five localities onthree Caribbean islands are morphologically analyzed and compared ina preliminary fashion with intertidal communities which I have observedelsewhere in the tropics A brief analysis of geographic distribution as afunction of habitat is also included

STUDY AREAS

Collections and ecological observations have been made at 23 localitieson rocky shores in Jamaica Curaltao St Maarten Saba Guadeloupeand the Caribbean coast of the Panama Canal Zone The five localitieschosen for morphological analysis represent typical Caribbean intertidalhabitats and are briefly described in the following paragraphs

Locality 1 Fort Point Jamaica-This locality lying just outside thenortheast entrance to Discovery Bay (see Fig 1) about 2 km north ofthe old site of the Discovery Bay Marine Laboratory on the north coastof Jamaica was visited on several occasions in January 1969 and manytimes from January to March 1970 Fort Point consists of a ruggedhoneycombed limestone terrace which at its seaward edge forms a slightlyundercut cliff 2-3 m high At the base of the cliff is a very narrow lowerplatform which slopes steeply away from the cliff for a distance of 1-3m before giving way to much deeper water Intertidal zonation is similarto that described by Lewis (1960) for cliff shores in Barbados exposed

354 Bulletin of Marine Science [23(2)

REEF CREST

DISCOVERY BAY Loe2 i_OLD

lABORATORY SITE

FIGURE 1 Map showing collecting localities in the vicinity of Discovery BayJamaica

to similarly heavy wave action The littoral fringe extends from the topof a zone of mosslike algae on the vertical face of the cliff about 150-180cm below the upper terrace to the seaward edge of the zone of Sesuviumportulacoides a succulent angiosperm on the upper terrace 3-4 m fromthe cliffs edge (see Table 1) Seaward portions of the Sesllvium zoneand the immediately adjacent upper terrace in the littoral fringe are oc-cupied by the littorinid Teetarius murieatus Three additional littorinids(Nodilittorina tubereulata Eehininus nodulosus and Littorina lineata)occur on the upper terrace but do not extend to the Sesuvium zonelike Tectarius Echininus and L lineata do not occur on the vertical faceof the cliff but Nodilittorina extends down into the zone of mosslike algaeThe cliff face above the zone of mosslike algae is occupied by Littorinaziczac L lineolata (the latter extending to higher levels than the former)Nerita versicolor and N peloronta The last-named species may oc-casionally be found on the seawardmost parts of the upper terrace Thezone of mosslike algae about 45 cm in vertical extent is generally shadedand presents an aspect quite different from that of the littoral fringe aboveThe rather smooth cliff face is quite uniformly covered with a mat of thesealgae on which may be found L ziczac L meleagris N peloronta Nversicolor the limpet Acmaea leucopleura the chiton Acanthopleuragranulata the limpetlike muricid Purpura patula and in lower portionsoccasional Cittarium pica (Trochidae) and Fissurella nodosa (Fissurel-lidae) Splashpools within the zone of mosslike algae and seaward littoralfringe support Littorina mespillum the neritid Puperita pupa the limpet

1973] Vermeij West Indian Molluscan Communities 355

ltllcoN

356 Bulletin of Marine Science [23(2)

Acmaea pustulata the high-spired snails Batillaria minima (Potamididae)and Planaxis lineatus and the crevice-dwelling bivalves Isognomon bicolorand Brachidontes dominguensis (= B exustus)

The innermost recess of the cliffs undercut lies just below the zone ofmosslike algae and supports Acmaea jamaicensis Fissurella 110dosa Fbarbadensis F barbouri P patula L meleagris C pica and A granulataThe dark-colored Nerita tessellata is sporadic where erosion of the cliffresults in locally horizontal surfaces in this zone but the species is notcommon at Fort Point The lower platform is thickly covered withLithothamnium and mats of the zoanthid Zoanthus sociatus interspersedwith encrusting colonies of the coral Porites astreoides Large fleshyalgae of the genera Sargassum and Turbinaria are abundant and towardsthe edge of the platform large sea fans (Gorgonia flabellum) and headsof the coral Diploria are left momentarily out of water in the troughs ofthe waves Echinoids are rare on the lower platform There is a richassemblage of molluscs on the lower platform including the turbinidAstraea caelata the conid Conus regius the muricid Morula nodulosathe columbellid Columbella mercatoria the trochid Cittarium pica thefasciolariids Leucozonia leucozonalis and L ocellata the chitons Acantho-pleura granulata and Chiton marmoratus and the limpets Acmaea jamai-censis Fissurella nodosa and F barbadensis The shells of most of thesespecies are heavily encrusted with calcareous algae

Locality 2 Discovery Bay Jamaica-This localjty lying just north of thefishermans beach on the east shore of Discovery Bay Jamaica (see Fig1) is relatively sheltered from heavy wave action It was visited numeroustimes in January 1969 and from January to March 1970 The sub-stratum consists of an eroded reef-limestone bench merging with a beachof coarse calcareous sand at the shoreward edge The rocks in the littoralfringe and higher intertidal zone are sand-scoured during storms whichoccur mainly in winter The sublittoral fringe supports a luxuriant growthof algae including Acanthophora Halimeda Sargassum Padina andValonia as well as encrusting corals including Porites astreoides Siderastrearadians Isophyllia sinuosa and Millepora In somewhat deeper waterisolated heads of Diploria and Acropora palmata may be found Severalechinoids (Echinometra lucunter Diadema antillarum Tripneustes ven-tricosus Lytechinus variegatus and Eucidaris tribuloides) abound in self-made cavities Few molluscs inhabit open rocky surfaces in the sublittoralfringe the most commonly encountered gastropods are Leucozonia leuco-zonalis Cerithium litteratum Cymatium nicobaricum and Fissurella bar-badensis while Astraea tecta and Ocenebra intermedia are less commonThe fauna of crevices and the undersurfaces of stones is large and variedincluding several gastropods (see Table 3) and the bivalves Arcopsisadamsi Arca zebra Barbatia sp Chama sp lsognomon radiatus and

1973] Vermeij West Indian Molluscan Communities 357occasional distorted Modiolus americanus A shallow bed of Thalassiaadjacent to and somewhat overlapping this locality was not included inthe morphological analysis

The littoral fringe supports very few animals small specimens ofTectarius muricatus Nodilittorina tuberculata Littorina lineata L lineolataand Nerita versicolor are occasional In the high intertidal zone largeindividuals of Nerita tessellata and N fulgurans are quite common bothon open surfaces and beneath stones Crevices in the intertidal oftensupport mats of the small bivalves Isognomon bicolor and Brachidontesdominguensis interspersed with the larger I radiatus and Pinctada radiataThe last species are more characteristic of subtidal surfaces

Locality 3 Boca Playa Canoa Curafao-This locality on the heavily wave-exposed north coast of Curaltaowas visited in September 1969 and againin April 1970 The shore consists of a very wide honeycombed limestoneterrace lying in the littoral fringe which at its seaward edge is succeededin stepwise fashion by two lower platforms The zonation pattern on thehighest terrace is similar to that at Fort Point except that Echininus isabsent Flatbottomed pools in the littoral fringe contain Littorina lineataL minima Nodilittorina tuberculata Planaxis nucleus and P lineatusThe zone of mosslike algae on vertical surfaces at the seaward edge ofthe highest platform supports the chiton Acanthopleura granulata thegastropods Acmaea leucopleura and Purpura patula and the crevice-dwelling bivalves Isognomon bicolor and Brachidontes dominguensis Lit-torina meleagris was not found N erita tesselata is abundant on the middleplatform together with P patula Thais rustica Cittarium pica Acmaeajamaicensis Fissurella nodosa F barbadensis and Acanthopleura granu-lata These species with the exception of Purpura and N tessellata con-tinue to the lowest platform where they are accompanied by Leucozoniaocellata and a few individuals of Conus mus The echinoids Echinometralucunter and Lytechinus variegatus are abundant on the two lower plat-forms A luxuriant algal growth including large plants of Sargassumoccurs on the lowest platform where occasional small encrusting coloniesof Porites astreoides are also encountered

Locality 4 Piscadera Baai Curafao-This locality lying in front of theCaraibisch Marien-Biologisch Instituut about 4 km northwest of Willemstadon the south coast of Curaltaowas visited in January 1968 September1969 and April 1970 The shore is a relatively sheltered beach of erodedcoral rubble Molluscs are absent from most of the littoral fringe exceptin its lowest portion from the mean high-water mark to about 30 emabove which is inhabited by Nerita versicolor Planaxis nucleus andLittorina lineolata Open surfaces below the water line support Acmaeapustulata N erita tessellata and the xancid Vasum capitellus The echinoids

358 Bulletin of Marine Science [23(2)

Diadema antillarum and Echinometra lucunter and the encrusting coralSiderastrea radians are abundant but there is no luxuriant growth ofmacroscopic algae The fauna of crevices and the undersurfaces of stonesis quite rich supporting several gastropods (see Table 5) and the bivalvesArcopsis adamsi Barbatia sp and Isognomon radiatus

Locality 5 Vieux Habitants Guadeloupe-This locality visited in Feb-ruary 1971 lies on the west coast of the island of Guadeloupe about 9km north of Basse Terre and 2 km south of Vieux Habitants near theHotel Rocroy The area investigated consists of a V-shaped cove oneither side of which are outcrops of exfoliating volcanic rock separated fromone another by a beach of fine black sand Both sides of the cove areexposed to rather heavy wave action The north side of the cove (Locality5A) is severely sand-scoured and supports a somewhat unusual epifaunaThe highest shore levels are occupied by small eroded individuals ofNodilittorina tuberculata Littorina lineata L ziczac and Nerita versicolorThe rocks in the high intertidal zone support l-cm2 patches of mosslikealgae among which occur Littorina meleagris and an apparently undescribedlimpet of the genus Acmaea whose shell has a smooth exterior radiallyornamented with black and white stripes At middle and lower intertidallevels on open surfaces are found Fissurella nimbosa Acmaea sp indetand Chiton marmoratus the last-named species being generally somewhatsheltered from the most severe wave action Crevices in the rocks at lowlevels contain rather smooth individuals of Thais rustica and the bivalveslsognomon bicolor and Brachidontes dominguensis Small growths ofSargassum occupy the most seaward parts of the outcrops

The south side of the cove (Locality 5B) is not subjected to sandabrasion From about 100 cm to 30 em above the upper limit of a zoneof mosslike algae are found small individuals of Tectarius muricatusNodilittorina tuberculata and Littorina lineata From the upper limitof mosslike algae to 30 cm above may be found L ziczac L lineolataNerita versicolor and N peloronta The uppermost reaches of the moss-like algal zone contain on open and semiprotected surfaces the gastropodsNerita tessellata Acmaea leucopleura L ziczac and Planaxis nucleusAn occasional individual of the undescribed Acmaea may also be foundAt lower levels occur the chitons Acanthopleura granulata and Chiton mar-moratus and the gastropods N tessellata Acmaea antillarum Fissurellanodosa F barbadensis Tegula excavata Cittarium pica Purpura patuaand strongly noded individuals of Thais rustica In contrast to the northside of the cove the echinoid Echinometra lucunter and the large barnacleTetraclita stalactifera are abundant at low intertidal levels on the southside and encrusting colonies of the fire coral Millepora occur in the sub-littoral fringe

1973] Vermeij West Indian Molluscan Communities 359

c

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AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

[23(2)352 Bulletin of Marine Science

INTRODUCTION

It is a common observation that terrestrial COmmUnItIesunder similarphysical regimes but in different geographic regions support species whichthough taxonomically unrelated are morphologically convergent and oftendivide up that habitat in similar ways (Richards 1952 Cody 1968 1970Mooney amp Dunn 1970 Leigh 1971) The reasons for this morphologicaland ecological convergence are not often known in detail but in generalreflect the way in which a particular body plan or group of body plansinteracts with the external physicochemical environment While geographi-cally isolated marine communities differ less strikingly from one anotherin a taxonomic sense than do many terrestrial communities morphologicaland ecological convergence within and between communities is neverthe-less apparent (Stephenson 1948 Thorson 1957 Jackson et ai 1971)Close examination of geographically isolated convergent communities oftenhowever reveals subtle but important differences in morphological composi-tion and ecological structure which may reflect the effects of historicalaccident Thus it is well known that mainland and island communitiesoften differ from one another in species diversity number and types ofpredators and in other ways as a result of patterns of dispersal and im-migration (MacArthur amp Wilson 1967 Leigh 1971) Consequentlyisland biotas become biologically unique relative to those of neighboringmainland areas and species living on the island tend to restrict theirgeographic ranges while adapting to the unique biotic and physical condi-tions at hand (MacArthur amp Wilson 1967 Vermeij 1972)

Intertidal molluscan communities are ideally suited for a study ofmorphological and ecological convergence between geographically andtemporally isolated communities Not only is the intertidal zone a con-veniently accessible and widely distributed habitat with clearly definedboundaries but the molluscan shell is geometrically relatively simple andtherefore amenable to comparative mathematical analysis The presentpaper is the first in a series dealing with morphological analyses of intertidalmolluscan communities under different physical conditions in the samebiogeographic region By analyzing the relation between form and habitatin the several Recent marine biogeographic regions it is hoped to lay thefoundation for the interpretation of similarities and differences betweenthe communities in these areas which are subjected to more-or-Iess-com-parable physicochemical conditions

Most previous literature on intertidal communities has been concernedwith the description of patterns of vertical and horizontal plant and animaldistribution on the shore (for reviews see Southward 1958 Knox 1960Sacchi 1969) A smaller body of literature has dealt with trophic structureand competitive interactions (see Paine 1966 1969 Connell 1961a 1961b1970 Batzli 1969) Although morphological studies have been carried

1973] Vermeij West Indian Molluscan Communities 353

out on particular species or groups of species of intertidal molluscs (egsee Staiger 1957 James 1968 Struhsaker 1968 Giesel 1970) almostno information exists on general morphological patterns among speciesin different habitats within one biogeographic region Most attempts thathave been made along these lines (Taylor 1968 Coomans 1969) havebeen qualitative and incidental and no rigorous morphological analyseswere undertaken Similarly while some data are available on faunisticand floristic differences between biogeographic regions (eg Ekman 1953Knox 1960 1963 Woodring 1966 Macnae 1968) no quantitative in-formation is available concerning differences and similarities in morphologyin comparable habitats in different geographic areas Nicol (1964a 1964b1965 1966 1967 1968) has made some general observations on mor-phological features of bivalves in different parts of the world but has notapproached the subject from an ecological point of view Stanley (1970)has made a valuable contribution to our understanding of the relation be-tween bivalve shell form and habitat but biogeographic differences werenot emphasized by him Elsewhere (Vermeij in prep) I have given anaccount of morphological patterns in high intertidal and supratidal gas-tropod assemblages suggesting that many properties of the shells oflittorinids neritids and limpets are adaptations to the rigorous conditionsat high shore levels

In the present paper the molluscan communities at five localities onthree Caribbean islands are morphologically analyzed and compared ina preliminary fashion with intertidal communities which I have observedelsewhere in the tropics A brief analysis of geographic distribution as afunction of habitat is also included

STUDY AREAS

Collections and ecological observations have been made at 23 localitieson rocky shores in Jamaica Curaltao St Maarten Saba Guadeloupeand the Caribbean coast of the Panama Canal Zone The five localitieschosen for morphological analysis represent typical Caribbean intertidalhabitats and are briefly described in the following paragraphs

Locality 1 Fort Point Jamaica-This locality lying just outside thenortheast entrance to Discovery Bay (see Fig 1) about 2 km north ofthe old site of the Discovery Bay Marine Laboratory on the north coastof Jamaica was visited on several occasions in January 1969 and manytimes from January to March 1970 Fort Point consists of a ruggedhoneycombed limestone terrace which at its seaward edge forms a slightlyundercut cliff 2-3 m high At the base of the cliff is a very narrow lowerplatform which slopes steeply away from the cliff for a distance of 1-3m before giving way to much deeper water Intertidal zonation is similarto that described by Lewis (1960) for cliff shores in Barbados exposed

354 Bulletin of Marine Science [23(2)

REEF CREST

DISCOVERY BAY Loe2 i_OLD

lABORATORY SITE

FIGURE 1 Map showing collecting localities in the vicinity of Discovery BayJamaica

to similarly heavy wave action The littoral fringe extends from the topof a zone of mosslike algae on the vertical face of the cliff about 150-180cm below the upper terrace to the seaward edge of the zone of Sesuviumportulacoides a succulent angiosperm on the upper terrace 3-4 m fromthe cliffs edge (see Table 1) Seaward portions of the Sesllvium zoneand the immediately adjacent upper terrace in the littoral fringe are oc-cupied by the littorinid Teetarius murieatus Three additional littorinids(Nodilittorina tubereulata Eehininus nodulosus and Littorina lineata)occur on the upper terrace but do not extend to the Sesuvium zonelike Tectarius Echininus and L lineata do not occur on the vertical faceof the cliff but Nodilittorina extends down into the zone of mosslike algaeThe cliff face above the zone of mosslike algae is occupied by Littorinaziczac L lineolata (the latter extending to higher levels than the former)Nerita versicolor and N peloronta The last-named species may oc-casionally be found on the seawardmost parts of the upper terrace Thezone of mosslike algae about 45 cm in vertical extent is generally shadedand presents an aspect quite different from that of the littoral fringe aboveThe rather smooth cliff face is quite uniformly covered with a mat of thesealgae on which may be found L ziczac L meleagris N peloronta Nversicolor the limpet Acmaea leucopleura the chiton Acanthopleuragranulata the limpetlike muricid Purpura patula and in lower portionsoccasional Cittarium pica (Trochidae) and Fissurella nodosa (Fissurel-lidae) Splashpools within the zone of mosslike algae and seaward littoralfringe support Littorina mespillum the neritid Puperita pupa the limpet

1973] Vermeij West Indian Molluscan Communities 355

ltllcoN

356 Bulletin of Marine Science [23(2)

Acmaea pustulata the high-spired snails Batillaria minima (Potamididae)and Planaxis lineatus and the crevice-dwelling bivalves Isognomon bicolorand Brachidontes dominguensis (= B exustus)

The innermost recess of the cliffs undercut lies just below the zone ofmosslike algae and supports Acmaea jamaicensis Fissurella 110dosa Fbarbadensis F barbouri P patula L meleagris C pica and A granulataThe dark-colored Nerita tessellata is sporadic where erosion of the cliffresults in locally horizontal surfaces in this zone but the species is notcommon at Fort Point The lower platform is thickly covered withLithothamnium and mats of the zoanthid Zoanthus sociatus interspersedwith encrusting colonies of the coral Porites astreoides Large fleshyalgae of the genera Sargassum and Turbinaria are abundant and towardsthe edge of the platform large sea fans (Gorgonia flabellum) and headsof the coral Diploria are left momentarily out of water in the troughs ofthe waves Echinoids are rare on the lower platform There is a richassemblage of molluscs on the lower platform including the turbinidAstraea caelata the conid Conus regius the muricid Morula nodulosathe columbellid Columbella mercatoria the trochid Cittarium pica thefasciolariids Leucozonia leucozonalis and L ocellata the chitons Acantho-pleura granulata and Chiton marmoratus and the limpets Acmaea jamai-censis Fissurella nodosa and F barbadensis The shells of most of thesespecies are heavily encrusted with calcareous algae

Locality 2 Discovery Bay Jamaica-This localjty lying just north of thefishermans beach on the east shore of Discovery Bay Jamaica (see Fig1) is relatively sheltered from heavy wave action It was visited numeroustimes in January 1969 and from January to March 1970 The sub-stratum consists of an eroded reef-limestone bench merging with a beachof coarse calcareous sand at the shoreward edge The rocks in the littoralfringe and higher intertidal zone are sand-scoured during storms whichoccur mainly in winter The sublittoral fringe supports a luxuriant growthof algae including Acanthophora Halimeda Sargassum Padina andValonia as well as encrusting corals including Porites astreoides Siderastrearadians Isophyllia sinuosa and Millepora In somewhat deeper waterisolated heads of Diploria and Acropora palmata may be found Severalechinoids (Echinometra lucunter Diadema antillarum Tripneustes ven-tricosus Lytechinus variegatus and Eucidaris tribuloides) abound in self-made cavities Few molluscs inhabit open rocky surfaces in the sublittoralfringe the most commonly encountered gastropods are Leucozonia leuco-zonalis Cerithium litteratum Cymatium nicobaricum and Fissurella bar-badensis while Astraea tecta and Ocenebra intermedia are less commonThe fauna of crevices and the undersurfaces of stones is large and variedincluding several gastropods (see Table 3) and the bivalves Arcopsisadamsi Arca zebra Barbatia sp Chama sp lsognomon radiatus and

1973] Vermeij West Indian Molluscan Communities 357occasional distorted Modiolus americanus A shallow bed of Thalassiaadjacent to and somewhat overlapping this locality was not included inthe morphological analysis

The littoral fringe supports very few animals small specimens ofTectarius muricatus Nodilittorina tuberculata Littorina lineata L lineolataand Nerita versicolor are occasional In the high intertidal zone largeindividuals of Nerita tessellata and N fulgurans are quite common bothon open surfaces and beneath stones Crevices in the intertidal oftensupport mats of the small bivalves Isognomon bicolor and Brachidontesdominguensis interspersed with the larger I radiatus and Pinctada radiataThe last species are more characteristic of subtidal surfaces

Locality 3 Boca Playa Canoa Curafao-This locality on the heavily wave-exposed north coast of Curaltaowas visited in September 1969 and againin April 1970 The shore consists of a very wide honeycombed limestoneterrace lying in the littoral fringe which at its seaward edge is succeededin stepwise fashion by two lower platforms The zonation pattern on thehighest terrace is similar to that at Fort Point except that Echininus isabsent Flatbottomed pools in the littoral fringe contain Littorina lineataL minima Nodilittorina tuberculata Planaxis nucleus and P lineatusThe zone of mosslike algae on vertical surfaces at the seaward edge ofthe highest platform supports the chiton Acanthopleura granulata thegastropods Acmaea leucopleura and Purpura patula and the crevice-dwelling bivalves Isognomon bicolor and Brachidontes dominguensis Lit-torina meleagris was not found N erita tesselata is abundant on the middleplatform together with P patula Thais rustica Cittarium pica Acmaeajamaicensis Fissurella nodosa F barbadensis and Acanthopleura granu-lata These species with the exception of Purpura and N tessellata con-tinue to the lowest platform where they are accompanied by Leucozoniaocellata and a few individuals of Conus mus The echinoids Echinometralucunter and Lytechinus variegatus are abundant on the two lower plat-forms A luxuriant algal growth including large plants of Sargassumoccurs on the lowest platform where occasional small encrusting coloniesof Porites astreoides are also encountered

Locality 4 Piscadera Baai Curafao-This locality lying in front of theCaraibisch Marien-Biologisch Instituut about 4 km northwest of Willemstadon the south coast of Curaltaowas visited in January 1968 September1969 and April 1970 The shore is a relatively sheltered beach of erodedcoral rubble Molluscs are absent from most of the littoral fringe exceptin its lowest portion from the mean high-water mark to about 30 emabove which is inhabited by Nerita versicolor Planaxis nucleus andLittorina lineolata Open surfaces below the water line support Acmaeapustulata N erita tessellata and the xancid Vasum capitellus The echinoids

358 Bulletin of Marine Science [23(2)

Diadema antillarum and Echinometra lucunter and the encrusting coralSiderastrea radians are abundant but there is no luxuriant growth ofmacroscopic algae The fauna of crevices and the undersurfaces of stonesis quite rich supporting several gastropods (see Table 5) and the bivalvesArcopsis adamsi Barbatia sp and Isognomon radiatus

Locality 5 Vieux Habitants Guadeloupe-This locality visited in Feb-ruary 1971 lies on the west coast of the island of Guadeloupe about 9km north of Basse Terre and 2 km south of Vieux Habitants near theHotel Rocroy The area investigated consists of a V-shaped cove oneither side of which are outcrops of exfoliating volcanic rock separated fromone another by a beach of fine black sand Both sides of the cove areexposed to rather heavy wave action The north side of the cove (Locality5A) is severely sand-scoured and supports a somewhat unusual epifaunaThe highest shore levels are occupied by small eroded individuals ofNodilittorina tuberculata Littorina lineata L ziczac and Nerita versicolorThe rocks in the high intertidal zone support l-cm2 patches of mosslikealgae among which occur Littorina meleagris and an apparently undescribedlimpet of the genus Acmaea whose shell has a smooth exterior radiallyornamented with black and white stripes At middle and lower intertidallevels on open surfaces are found Fissurella nimbosa Acmaea sp indetand Chiton marmoratus the last-named species being generally somewhatsheltered from the most severe wave action Crevices in the rocks at lowlevels contain rather smooth individuals of Thais rustica and the bivalveslsognomon bicolor and Brachidontes dominguensis Small growths ofSargassum occupy the most seaward parts of the outcrops

The south side of the cove (Locality 5B) is not subjected to sandabrasion From about 100 cm to 30 em above the upper limit of a zoneof mosslike algae are found small individuals of Tectarius muricatusNodilittorina tuberculata and Littorina lineata From the upper limitof mosslike algae to 30 cm above may be found L ziczac L lineolataNerita versicolor and N peloronta The uppermost reaches of the moss-like algal zone contain on open and semiprotected surfaces the gastropodsNerita tessellata Acmaea leucopleura L ziczac and Planaxis nucleusAn occasional individual of the undescribed Acmaea may also be foundAt lower levels occur the chitons Acanthopleura granulata and Chiton mar-moratus and the gastropods N tessellata Acmaea antillarum Fissurellanodosa F barbadensis Tegula excavata Cittarium pica Purpura patuaand strongly noded individuals of Thais rustica In contrast to the northside of the cove the echinoid Echinometra lucunter and the large barnacleTetraclita stalactifera are abundant at low intertidal levels on the southside and encrusting colonies of the fire coral Millepora occur in the sub-littoral fringe

1973] Vermeij West Indian Molluscan Communities 359

c

B

AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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364 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 353

out on particular species or groups of species of intertidal molluscs (egsee Staiger 1957 James 1968 Struhsaker 1968 Giesel 1970) almostno information exists on general morphological patterns among speciesin different habitats within one biogeographic region Most attempts thathave been made along these lines (Taylor 1968 Coomans 1969) havebeen qualitative and incidental and no rigorous morphological analyseswere undertaken Similarly while some data are available on faunisticand floristic differences between biogeographic regions (eg Ekman 1953Knox 1960 1963 Woodring 1966 Macnae 1968) no quantitative in-formation is available concerning differences and similarities in morphologyin comparable habitats in different geographic areas Nicol (1964a 1964b1965 1966 1967 1968) has made some general observations on mor-phological features of bivalves in different parts of the world but has notapproached the subject from an ecological point of view Stanley (1970)has made a valuable contribution to our understanding of the relation be-tween bivalve shell form and habitat but biogeographic differences werenot emphasized by him Elsewhere (Vermeij in prep) I have given anaccount of morphological patterns in high intertidal and supratidal gas-tropod assemblages suggesting that many properties of the shells oflittorinids neritids and limpets are adaptations to the rigorous conditionsat high shore levels

In the present paper the molluscan communities at five localities onthree Caribbean islands are morphologically analyzed and compared ina preliminary fashion with intertidal communities which I have observedelsewhere in the tropics A brief analysis of geographic distribution as afunction of habitat is also included

STUDY AREAS

Collections and ecological observations have been made at 23 localitieson rocky shores in Jamaica Curaltao St Maarten Saba Guadeloupeand the Caribbean coast of the Panama Canal Zone The five localitieschosen for morphological analysis represent typical Caribbean intertidalhabitats and are briefly described in the following paragraphs

Locality 1 Fort Point Jamaica-This locality lying just outside thenortheast entrance to Discovery Bay (see Fig 1) about 2 km north ofthe old site of the Discovery Bay Marine Laboratory on the north coastof Jamaica was visited on several occasions in January 1969 and manytimes from January to March 1970 Fort Point consists of a ruggedhoneycombed limestone terrace which at its seaward edge forms a slightlyundercut cliff 2-3 m high At the base of the cliff is a very narrow lowerplatform which slopes steeply away from the cliff for a distance of 1-3m before giving way to much deeper water Intertidal zonation is similarto that described by Lewis (1960) for cliff shores in Barbados exposed

354 Bulletin of Marine Science [23(2)

REEF CREST

DISCOVERY BAY Loe2 i_OLD

lABORATORY SITE

FIGURE 1 Map showing collecting localities in the vicinity of Discovery BayJamaica

to similarly heavy wave action The littoral fringe extends from the topof a zone of mosslike algae on the vertical face of the cliff about 150-180cm below the upper terrace to the seaward edge of the zone of Sesuviumportulacoides a succulent angiosperm on the upper terrace 3-4 m fromthe cliffs edge (see Table 1) Seaward portions of the Sesllvium zoneand the immediately adjacent upper terrace in the littoral fringe are oc-cupied by the littorinid Teetarius murieatus Three additional littorinids(Nodilittorina tubereulata Eehininus nodulosus and Littorina lineata)occur on the upper terrace but do not extend to the Sesuvium zonelike Tectarius Echininus and L lineata do not occur on the vertical faceof the cliff but Nodilittorina extends down into the zone of mosslike algaeThe cliff face above the zone of mosslike algae is occupied by Littorinaziczac L lineolata (the latter extending to higher levels than the former)Nerita versicolor and N peloronta The last-named species may oc-casionally be found on the seawardmost parts of the upper terrace Thezone of mosslike algae about 45 cm in vertical extent is generally shadedand presents an aspect quite different from that of the littoral fringe aboveThe rather smooth cliff face is quite uniformly covered with a mat of thesealgae on which may be found L ziczac L meleagris N peloronta Nversicolor the limpet Acmaea leucopleura the chiton Acanthopleuragranulata the limpetlike muricid Purpura patula and in lower portionsoccasional Cittarium pica (Trochidae) and Fissurella nodosa (Fissurel-lidae) Splashpools within the zone of mosslike algae and seaward littoralfringe support Littorina mespillum the neritid Puperita pupa the limpet

1973] Vermeij West Indian Molluscan Communities 355

ltllcoN

356 Bulletin of Marine Science [23(2)

Acmaea pustulata the high-spired snails Batillaria minima (Potamididae)and Planaxis lineatus and the crevice-dwelling bivalves Isognomon bicolorand Brachidontes dominguensis (= B exustus)

The innermost recess of the cliffs undercut lies just below the zone ofmosslike algae and supports Acmaea jamaicensis Fissurella 110dosa Fbarbadensis F barbouri P patula L meleagris C pica and A granulataThe dark-colored Nerita tessellata is sporadic where erosion of the cliffresults in locally horizontal surfaces in this zone but the species is notcommon at Fort Point The lower platform is thickly covered withLithothamnium and mats of the zoanthid Zoanthus sociatus interspersedwith encrusting colonies of the coral Porites astreoides Large fleshyalgae of the genera Sargassum and Turbinaria are abundant and towardsthe edge of the platform large sea fans (Gorgonia flabellum) and headsof the coral Diploria are left momentarily out of water in the troughs ofthe waves Echinoids are rare on the lower platform There is a richassemblage of molluscs on the lower platform including the turbinidAstraea caelata the conid Conus regius the muricid Morula nodulosathe columbellid Columbella mercatoria the trochid Cittarium pica thefasciolariids Leucozonia leucozonalis and L ocellata the chitons Acantho-pleura granulata and Chiton marmoratus and the limpets Acmaea jamai-censis Fissurella nodosa and F barbadensis The shells of most of thesespecies are heavily encrusted with calcareous algae

Locality 2 Discovery Bay Jamaica-This localjty lying just north of thefishermans beach on the east shore of Discovery Bay Jamaica (see Fig1) is relatively sheltered from heavy wave action It was visited numeroustimes in January 1969 and from January to March 1970 The sub-stratum consists of an eroded reef-limestone bench merging with a beachof coarse calcareous sand at the shoreward edge The rocks in the littoralfringe and higher intertidal zone are sand-scoured during storms whichoccur mainly in winter The sublittoral fringe supports a luxuriant growthof algae including Acanthophora Halimeda Sargassum Padina andValonia as well as encrusting corals including Porites astreoides Siderastrearadians Isophyllia sinuosa and Millepora In somewhat deeper waterisolated heads of Diploria and Acropora palmata may be found Severalechinoids (Echinometra lucunter Diadema antillarum Tripneustes ven-tricosus Lytechinus variegatus and Eucidaris tribuloides) abound in self-made cavities Few molluscs inhabit open rocky surfaces in the sublittoralfringe the most commonly encountered gastropods are Leucozonia leuco-zonalis Cerithium litteratum Cymatium nicobaricum and Fissurella bar-badensis while Astraea tecta and Ocenebra intermedia are less commonThe fauna of crevices and the undersurfaces of stones is large and variedincluding several gastropods (see Table 3) and the bivalves Arcopsisadamsi Arca zebra Barbatia sp Chama sp lsognomon radiatus and

1973] Vermeij West Indian Molluscan Communities 357occasional distorted Modiolus americanus A shallow bed of Thalassiaadjacent to and somewhat overlapping this locality was not included inthe morphological analysis

The littoral fringe supports very few animals small specimens ofTectarius muricatus Nodilittorina tuberculata Littorina lineata L lineolataand Nerita versicolor are occasional In the high intertidal zone largeindividuals of Nerita tessellata and N fulgurans are quite common bothon open surfaces and beneath stones Crevices in the intertidal oftensupport mats of the small bivalves Isognomon bicolor and Brachidontesdominguensis interspersed with the larger I radiatus and Pinctada radiataThe last species are more characteristic of subtidal surfaces

Locality 3 Boca Playa Canoa Curafao-This locality on the heavily wave-exposed north coast of Curaltaowas visited in September 1969 and againin April 1970 The shore consists of a very wide honeycombed limestoneterrace lying in the littoral fringe which at its seaward edge is succeededin stepwise fashion by two lower platforms The zonation pattern on thehighest terrace is similar to that at Fort Point except that Echininus isabsent Flatbottomed pools in the littoral fringe contain Littorina lineataL minima Nodilittorina tuberculata Planaxis nucleus and P lineatusThe zone of mosslike algae on vertical surfaces at the seaward edge ofthe highest platform supports the chiton Acanthopleura granulata thegastropods Acmaea leucopleura and Purpura patula and the crevice-dwelling bivalves Isognomon bicolor and Brachidontes dominguensis Lit-torina meleagris was not found N erita tesselata is abundant on the middleplatform together with P patula Thais rustica Cittarium pica Acmaeajamaicensis Fissurella nodosa F barbadensis and Acanthopleura granu-lata These species with the exception of Purpura and N tessellata con-tinue to the lowest platform where they are accompanied by Leucozoniaocellata and a few individuals of Conus mus The echinoids Echinometralucunter and Lytechinus variegatus are abundant on the two lower plat-forms A luxuriant algal growth including large plants of Sargassumoccurs on the lowest platform where occasional small encrusting coloniesof Porites astreoides are also encountered

Locality 4 Piscadera Baai Curafao-This locality lying in front of theCaraibisch Marien-Biologisch Instituut about 4 km northwest of Willemstadon the south coast of Curaltaowas visited in January 1968 September1969 and April 1970 The shore is a relatively sheltered beach of erodedcoral rubble Molluscs are absent from most of the littoral fringe exceptin its lowest portion from the mean high-water mark to about 30 emabove which is inhabited by Nerita versicolor Planaxis nucleus andLittorina lineolata Open surfaces below the water line support Acmaeapustulata N erita tessellata and the xancid Vasum capitellus The echinoids

358 Bulletin of Marine Science [23(2)

Diadema antillarum and Echinometra lucunter and the encrusting coralSiderastrea radians are abundant but there is no luxuriant growth ofmacroscopic algae The fauna of crevices and the undersurfaces of stonesis quite rich supporting several gastropods (see Table 5) and the bivalvesArcopsis adamsi Barbatia sp and Isognomon radiatus

Locality 5 Vieux Habitants Guadeloupe-This locality visited in Feb-ruary 1971 lies on the west coast of the island of Guadeloupe about 9km north of Basse Terre and 2 km south of Vieux Habitants near theHotel Rocroy The area investigated consists of a V-shaped cove oneither side of which are outcrops of exfoliating volcanic rock separated fromone another by a beach of fine black sand Both sides of the cove areexposed to rather heavy wave action The north side of the cove (Locality5A) is severely sand-scoured and supports a somewhat unusual epifaunaThe highest shore levels are occupied by small eroded individuals ofNodilittorina tuberculata Littorina lineata L ziczac and Nerita versicolorThe rocks in the high intertidal zone support l-cm2 patches of mosslikealgae among which occur Littorina meleagris and an apparently undescribedlimpet of the genus Acmaea whose shell has a smooth exterior radiallyornamented with black and white stripes At middle and lower intertidallevels on open surfaces are found Fissurella nimbosa Acmaea sp indetand Chiton marmoratus the last-named species being generally somewhatsheltered from the most severe wave action Crevices in the rocks at lowlevels contain rather smooth individuals of Thais rustica and the bivalveslsognomon bicolor and Brachidontes dominguensis Small growths ofSargassum occupy the most seaward parts of the outcrops

The south side of the cove (Locality 5B) is not subjected to sandabrasion From about 100 cm to 30 em above the upper limit of a zoneof mosslike algae are found small individuals of Tectarius muricatusNodilittorina tuberculata and Littorina lineata From the upper limitof mosslike algae to 30 cm above may be found L ziczac L lineolataNerita versicolor and N peloronta The uppermost reaches of the moss-like algal zone contain on open and semiprotected surfaces the gastropodsNerita tessellata Acmaea leucopleura L ziczac and Planaxis nucleusAn occasional individual of the undescribed Acmaea may also be foundAt lower levels occur the chitons Acanthopleura granulata and Chiton mar-moratus and the gastropods N tessellata Acmaea antillarum Fissurellanodosa F barbadensis Tegula excavata Cittarium pica Purpura patuaand strongly noded individuals of Thais rustica In contrast to the northside of the cove the echinoid Echinometra lucunter and the large barnacleTetraclita stalactifera are abundant at low intertidal levels on the southside and encrusting colonies of the fire coral Millepora occur in the sub-littoral fringe

1973] Vermeij West Indian Molluscan Communities 359

c

B

AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

354 Bulletin of Marine Science [23(2)

REEF CREST

DISCOVERY BAY Loe2 i_OLD

lABORATORY SITE

FIGURE 1 Map showing collecting localities in the vicinity of Discovery BayJamaica

to similarly heavy wave action The littoral fringe extends from the topof a zone of mosslike algae on the vertical face of the cliff about 150-180cm below the upper terrace to the seaward edge of the zone of Sesuviumportulacoides a succulent angiosperm on the upper terrace 3-4 m fromthe cliffs edge (see Table 1) Seaward portions of the Sesllvium zoneand the immediately adjacent upper terrace in the littoral fringe are oc-cupied by the littorinid Teetarius murieatus Three additional littorinids(Nodilittorina tubereulata Eehininus nodulosus and Littorina lineata)occur on the upper terrace but do not extend to the Sesuvium zonelike Tectarius Echininus and L lineata do not occur on the vertical faceof the cliff but Nodilittorina extends down into the zone of mosslike algaeThe cliff face above the zone of mosslike algae is occupied by Littorinaziczac L lineolata (the latter extending to higher levels than the former)Nerita versicolor and N peloronta The last-named species may oc-casionally be found on the seawardmost parts of the upper terrace Thezone of mosslike algae about 45 cm in vertical extent is generally shadedand presents an aspect quite different from that of the littoral fringe aboveThe rather smooth cliff face is quite uniformly covered with a mat of thesealgae on which may be found L ziczac L meleagris N peloronta Nversicolor the limpet Acmaea leucopleura the chiton Acanthopleuragranulata the limpetlike muricid Purpura patula and in lower portionsoccasional Cittarium pica (Trochidae) and Fissurella nodosa (Fissurel-lidae) Splashpools within the zone of mosslike algae and seaward littoralfringe support Littorina mespillum the neritid Puperita pupa the limpet

1973] Vermeij West Indian Molluscan Communities 355

ltllcoN

356 Bulletin of Marine Science [23(2)

Acmaea pustulata the high-spired snails Batillaria minima (Potamididae)and Planaxis lineatus and the crevice-dwelling bivalves Isognomon bicolorand Brachidontes dominguensis (= B exustus)

The innermost recess of the cliffs undercut lies just below the zone ofmosslike algae and supports Acmaea jamaicensis Fissurella 110dosa Fbarbadensis F barbouri P patula L meleagris C pica and A granulataThe dark-colored Nerita tessellata is sporadic where erosion of the cliffresults in locally horizontal surfaces in this zone but the species is notcommon at Fort Point The lower platform is thickly covered withLithothamnium and mats of the zoanthid Zoanthus sociatus interspersedwith encrusting colonies of the coral Porites astreoides Large fleshyalgae of the genera Sargassum and Turbinaria are abundant and towardsthe edge of the platform large sea fans (Gorgonia flabellum) and headsof the coral Diploria are left momentarily out of water in the troughs ofthe waves Echinoids are rare on the lower platform There is a richassemblage of molluscs on the lower platform including the turbinidAstraea caelata the conid Conus regius the muricid Morula nodulosathe columbellid Columbella mercatoria the trochid Cittarium pica thefasciolariids Leucozonia leucozonalis and L ocellata the chitons Acantho-pleura granulata and Chiton marmoratus and the limpets Acmaea jamai-censis Fissurella nodosa and F barbadensis The shells of most of thesespecies are heavily encrusted with calcareous algae

Locality 2 Discovery Bay Jamaica-This localjty lying just north of thefishermans beach on the east shore of Discovery Bay Jamaica (see Fig1) is relatively sheltered from heavy wave action It was visited numeroustimes in January 1969 and from January to March 1970 The sub-stratum consists of an eroded reef-limestone bench merging with a beachof coarse calcareous sand at the shoreward edge The rocks in the littoralfringe and higher intertidal zone are sand-scoured during storms whichoccur mainly in winter The sublittoral fringe supports a luxuriant growthof algae including Acanthophora Halimeda Sargassum Padina andValonia as well as encrusting corals including Porites astreoides Siderastrearadians Isophyllia sinuosa and Millepora In somewhat deeper waterisolated heads of Diploria and Acropora palmata may be found Severalechinoids (Echinometra lucunter Diadema antillarum Tripneustes ven-tricosus Lytechinus variegatus and Eucidaris tribuloides) abound in self-made cavities Few molluscs inhabit open rocky surfaces in the sublittoralfringe the most commonly encountered gastropods are Leucozonia leuco-zonalis Cerithium litteratum Cymatium nicobaricum and Fissurella bar-badensis while Astraea tecta and Ocenebra intermedia are less commonThe fauna of crevices and the undersurfaces of stones is large and variedincluding several gastropods (see Table 3) and the bivalves Arcopsisadamsi Arca zebra Barbatia sp Chama sp lsognomon radiatus and

1973] Vermeij West Indian Molluscan Communities 357occasional distorted Modiolus americanus A shallow bed of Thalassiaadjacent to and somewhat overlapping this locality was not included inthe morphological analysis

The littoral fringe supports very few animals small specimens ofTectarius muricatus Nodilittorina tuberculata Littorina lineata L lineolataand Nerita versicolor are occasional In the high intertidal zone largeindividuals of Nerita tessellata and N fulgurans are quite common bothon open surfaces and beneath stones Crevices in the intertidal oftensupport mats of the small bivalves Isognomon bicolor and Brachidontesdominguensis interspersed with the larger I radiatus and Pinctada radiataThe last species are more characteristic of subtidal surfaces

Locality 3 Boca Playa Canoa Curafao-This locality on the heavily wave-exposed north coast of Curaltaowas visited in September 1969 and againin April 1970 The shore consists of a very wide honeycombed limestoneterrace lying in the littoral fringe which at its seaward edge is succeededin stepwise fashion by two lower platforms The zonation pattern on thehighest terrace is similar to that at Fort Point except that Echininus isabsent Flatbottomed pools in the littoral fringe contain Littorina lineataL minima Nodilittorina tuberculata Planaxis nucleus and P lineatusThe zone of mosslike algae on vertical surfaces at the seaward edge ofthe highest platform supports the chiton Acanthopleura granulata thegastropods Acmaea leucopleura and Purpura patula and the crevice-dwelling bivalves Isognomon bicolor and Brachidontes dominguensis Lit-torina meleagris was not found N erita tesselata is abundant on the middleplatform together with P patula Thais rustica Cittarium pica Acmaeajamaicensis Fissurella nodosa F barbadensis and Acanthopleura granu-lata These species with the exception of Purpura and N tessellata con-tinue to the lowest platform where they are accompanied by Leucozoniaocellata and a few individuals of Conus mus The echinoids Echinometralucunter and Lytechinus variegatus are abundant on the two lower plat-forms A luxuriant algal growth including large plants of Sargassumoccurs on the lowest platform where occasional small encrusting coloniesof Porites astreoides are also encountered

Locality 4 Piscadera Baai Curafao-This locality lying in front of theCaraibisch Marien-Biologisch Instituut about 4 km northwest of Willemstadon the south coast of Curaltaowas visited in January 1968 September1969 and April 1970 The shore is a relatively sheltered beach of erodedcoral rubble Molluscs are absent from most of the littoral fringe exceptin its lowest portion from the mean high-water mark to about 30 emabove which is inhabited by Nerita versicolor Planaxis nucleus andLittorina lineolata Open surfaces below the water line support Acmaeapustulata N erita tessellata and the xancid Vasum capitellus The echinoids

358 Bulletin of Marine Science [23(2)

Diadema antillarum and Echinometra lucunter and the encrusting coralSiderastrea radians are abundant but there is no luxuriant growth ofmacroscopic algae The fauna of crevices and the undersurfaces of stonesis quite rich supporting several gastropods (see Table 5) and the bivalvesArcopsis adamsi Barbatia sp and Isognomon radiatus

Locality 5 Vieux Habitants Guadeloupe-This locality visited in Feb-ruary 1971 lies on the west coast of the island of Guadeloupe about 9km north of Basse Terre and 2 km south of Vieux Habitants near theHotel Rocroy The area investigated consists of a V-shaped cove oneither side of which are outcrops of exfoliating volcanic rock separated fromone another by a beach of fine black sand Both sides of the cove areexposed to rather heavy wave action The north side of the cove (Locality5A) is severely sand-scoured and supports a somewhat unusual epifaunaThe highest shore levels are occupied by small eroded individuals ofNodilittorina tuberculata Littorina lineata L ziczac and Nerita versicolorThe rocks in the high intertidal zone support l-cm2 patches of mosslikealgae among which occur Littorina meleagris and an apparently undescribedlimpet of the genus Acmaea whose shell has a smooth exterior radiallyornamented with black and white stripes At middle and lower intertidallevels on open surfaces are found Fissurella nimbosa Acmaea sp indetand Chiton marmoratus the last-named species being generally somewhatsheltered from the most severe wave action Crevices in the rocks at lowlevels contain rather smooth individuals of Thais rustica and the bivalveslsognomon bicolor and Brachidontes dominguensis Small growths ofSargassum occupy the most seaward parts of the outcrops

The south side of the cove (Locality 5B) is not subjected to sandabrasion From about 100 cm to 30 em above the upper limit of a zoneof mosslike algae are found small individuals of Tectarius muricatusNodilittorina tuberculata and Littorina lineata From the upper limitof mosslike algae to 30 cm above may be found L ziczac L lineolataNerita versicolor and N peloronta The uppermost reaches of the moss-like algal zone contain on open and semiprotected surfaces the gastropodsNerita tessellata Acmaea leucopleura L ziczac and Planaxis nucleusAn occasional individual of the undescribed Acmaea may also be foundAt lower levels occur the chitons Acanthopleura granulata and Chiton mar-moratus and the gastropods N tessellata Acmaea antillarum Fissurellanodosa F barbadensis Tegula excavata Cittarium pica Purpura patuaand strongly noded individuals of Thais rustica In contrast to the northside of the cove the echinoid Echinometra lucunter and the large barnacleTetraclita stalactifera are abundant at low intertidal levels on the southside and encrusting colonies of the fire coral Millepora occur in the sub-littoral fringe

1973] Vermeij West Indian Molluscan Communities 359

c

B

AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 355

ltllcoN

356 Bulletin of Marine Science [23(2)

Acmaea pustulata the high-spired snails Batillaria minima (Potamididae)and Planaxis lineatus and the crevice-dwelling bivalves Isognomon bicolorand Brachidontes dominguensis (= B exustus)

The innermost recess of the cliffs undercut lies just below the zone ofmosslike algae and supports Acmaea jamaicensis Fissurella 110dosa Fbarbadensis F barbouri P patula L meleagris C pica and A granulataThe dark-colored Nerita tessellata is sporadic where erosion of the cliffresults in locally horizontal surfaces in this zone but the species is notcommon at Fort Point The lower platform is thickly covered withLithothamnium and mats of the zoanthid Zoanthus sociatus interspersedwith encrusting colonies of the coral Porites astreoides Large fleshyalgae of the genera Sargassum and Turbinaria are abundant and towardsthe edge of the platform large sea fans (Gorgonia flabellum) and headsof the coral Diploria are left momentarily out of water in the troughs ofthe waves Echinoids are rare on the lower platform There is a richassemblage of molluscs on the lower platform including the turbinidAstraea caelata the conid Conus regius the muricid Morula nodulosathe columbellid Columbella mercatoria the trochid Cittarium pica thefasciolariids Leucozonia leucozonalis and L ocellata the chitons Acantho-pleura granulata and Chiton marmoratus and the limpets Acmaea jamai-censis Fissurella nodosa and F barbadensis The shells of most of thesespecies are heavily encrusted with calcareous algae

Locality 2 Discovery Bay Jamaica-This localjty lying just north of thefishermans beach on the east shore of Discovery Bay Jamaica (see Fig1) is relatively sheltered from heavy wave action It was visited numeroustimes in January 1969 and from January to March 1970 The sub-stratum consists of an eroded reef-limestone bench merging with a beachof coarse calcareous sand at the shoreward edge The rocks in the littoralfringe and higher intertidal zone are sand-scoured during storms whichoccur mainly in winter The sublittoral fringe supports a luxuriant growthof algae including Acanthophora Halimeda Sargassum Padina andValonia as well as encrusting corals including Porites astreoides Siderastrearadians Isophyllia sinuosa and Millepora In somewhat deeper waterisolated heads of Diploria and Acropora palmata may be found Severalechinoids (Echinometra lucunter Diadema antillarum Tripneustes ven-tricosus Lytechinus variegatus and Eucidaris tribuloides) abound in self-made cavities Few molluscs inhabit open rocky surfaces in the sublittoralfringe the most commonly encountered gastropods are Leucozonia leuco-zonalis Cerithium litteratum Cymatium nicobaricum and Fissurella bar-badensis while Astraea tecta and Ocenebra intermedia are less commonThe fauna of crevices and the undersurfaces of stones is large and variedincluding several gastropods (see Table 3) and the bivalves Arcopsisadamsi Arca zebra Barbatia sp Chama sp lsognomon radiatus and

1973] Vermeij West Indian Molluscan Communities 357occasional distorted Modiolus americanus A shallow bed of Thalassiaadjacent to and somewhat overlapping this locality was not included inthe morphological analysis

The littoral fringe supports very few animals small specimens ofTectarius muricatus Nodilittorina tuberculata Littorina lineata L lineolataand Nerita versicolor are occasional In the high intertidal zone largeindividuals of Nerita tessellata and N fulgurans are quite common bothon open surfaces and beneath stones Crevices in the intertidal oftensupport mats of the small bivalves Isognomon bicolor and Brachidontesdominguensis interspersed with the larger I radiatus and Pinctada radiataThe last species are more characteristic of subtidal surfaces

Locality 3 Boca Playa Canoa Curafao-This locality on the heavily wave-exposed north coast of Curaltaowas visited in September 1969 and againin April 1970 The shore consists of a very wide honeycombed limestoneterrace lying in the littoral fringe which at its seaward edge is succeededin stepwise fashion by two lower platforms The zonation pattern on thehighest terrace is similar to that at Fort Point except that Echininus isabsent Flatbottomed pools in the littoral fringe contain Littorina lineataL minima Nodilittorina tuberculata Planaxis nucleus and P lineatusThe zone of mosslike algae on vertical surfaces at the seaward edge ofthe highest platform supports the chiton Acanthopleura granulata thegastropods Acmaea leucopleura and Purpura patula and the crevice-dwelling bivalves Isognomon bicolor and Brachidontes dominguensis Lit-torina meleagris was not found N erita tesselata is abundant on the middleplatform together with P patula Thais rustica Cittarium pica Acmaeajamaicensis Fissurella nodosa F barbadensis and Acanthopleura granu-lata These species with the exception of Purpura and N tessellata con-tinue to the lowest platform where they are accompanied by Leucozoniaocellata and a few individuals of Conus mus The echinoids Echinometralucunter and Lytechinus variegatus are abundant on the two lower plat-forms A luxuriant algal growth including large plants of Sargassumoccurs on the lowest platform where occasional small encrusting coloniesof Porites astreoides are also encountered

Locality 4 Piscadera Baai Curafao-This locality lying in front of theCaraibisch Marien-Biologisch Instituut about 4 km northwest of Willemstadon the south coast of Curaltaowas visited in January 1968 September1969 and April 1970 The shore is a relatively sheltered beach of erodedcoral rubble Molluscs are absent from most of the littoral fringe exceptin its lowest portion from the mean high-water mark to about 30 emabove which is inhabited by Nerita versicolor Planaxis nucleus andLittorina lineolata Open surfaces below the water line support Acmaeapustulata N erita tessellata and the xancid Vasum capitellus The echinoids

358 Bulletin of Marine Science [23(2)

Diadema antillarum and Echinometra lucunter and the encrusting coralSiderastrea radians are abundant but there is no luxuriant growth ofmacroscopic algae The fauna of crevices and the undersurfaces of stonesis quite rich supporting several gastropods (see Table 5) and the bivalvesArcopsis adamsi Barbatia sp and Isognomon radiatus

Locality 5 Vieux Habitants Guadeloupe-This locality visited in Feb-ruary 1971 lies on the west coast of the island of Guadeloupe about 9km north of Basse Terre and 2 km south of Vieux Habitants near theHotel Rocroy The area investigated consists of a V-shaped cove oneither side of which are outcrops of exfoliating volcanic rock separated fromone another by a beach of fine black sand Both sides of the cove areexposed to rather heavy wave action The north side of the cove (Locality5A) is severely sand-scoured and supports a somewhat unusual epifaunaThe highest shore levels are occupied by small eroded individuals ofNodilittorina tuberculata Littorina lineata L ziczac and Nerita versicolorThe rocks in the high intertidal zone support l-cm2 patches of mosslikealgae among which occur Littorina meleagris and an apparently undescribedlimpet of the genus Acmaea whose shell has a smooth exterior radiallyornamented with black and white stripes At middle and lower intertidallevels on open surfaces are found Fissurella nimbosa Acmaea sp indetand Chiton marmoratus the last-named species being generally somewhatsheltered from the most severe wave action Crevices in the rocks at lowlevels contain rather smooth individuals of Thais rustica and the bivalveslsognomon bicolor and Brachidontes dominguensis Small growths ofSargassum occupy the most seaward parts of the outcrops

The south side of the cove (Locality 5B) is not subjected to sandabrasion From about 100 cm to 30 em above the upper limit of a zoneof mosslike algae are found small individuals of Tectarius muricatusNodilittorina tuberculata and Littorina lineata From the upper limitof mosslike algae to 30 cm above may be found L ziczac L lineolataNerita versicolor and N peloronta The uppermost reaches of the moss-like algal zone contain on open and semiprotected surfaces the gastropodsNerita tessellata Acmaea leucopleura L ziczac and Planaxis nucleusAn occasional individual of the undescribed Acmaea may also be foundAt lower levels occur the chitons Acanthopleura granulata and Chiton mar-moratus and the gastropods N tessellata Acmaea antillarum Fissurellanodosa F barbadensis Tegula excavata Cittarium pica Purpura patuaand strongly noded individuals of Thais rustica In contrast to the northside of the cove the echinoid Echinometra lucunter and the large barnacleTetraclita stalactifera are abundant at low intertidal levels on the southside and encrusting colonies of the fire coral Millepora occur in the sub-littoral fringe

1973] Vermeij West Indian Molluscan Communities 359

c

B

AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 375

tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

356 Bulletin of Marine Science [23(2)

Acmaea pustulata the high-spired snails Batillaria minima (Potamididae)and Planaxis lineatus and the crevice-dwelling bivalves Isognomon bicolorand Brachidontes dominguensis (= B exustus)

The innermost recess of the cliffs undercut lies just below the zone ofmosslike algae and supports Acmaea jamaicensis Fissurella 110dosa Fbarbadensis F barbouri P patula L meleagris C pica and A granulataThe dark-colored Nerita tessellata is sporadic where erosion of the cliffresults in locally horizontal surfaces in this zone but the species is notcommon at Fort Point The lower platform is thickly covered withLithothamnium and mats of the zoanthid Zoanthus sociatus interspersedwith encrusting colonies of the coral Porites astreoides Large fleshyalgae of the genera Sargassum and Turbinaria are abundant and towardsthe edge of the platform large sea fans (Gorgonia flabellum) and headsof the coral Diploria are left momentarily out of water in the troughs ofthe waves Echinoids are rare on the lower platform There is a richassemblage of molluscs on the lower platform including the turbinidAstraea caelata the conid Conus regius the muricid Morula nodulosathe columbellid Columbella mercatoria the trochid Cittarium pica thefasciolariids Leucozonia leucozonalis and L ocellata the chitons Acantho-pleura granulata and Chiton marmoratus and the limpets Acmaea jamai-censis Fissurella nodosa and F barbadensis The shells of most of thesespecies are heavily encrusted with calcareous algae

Locality 2 Discovery Bay Jamaica-This localjty lying just north of thefishermans beach on the east shore of Discovery Bay Jamaica (see Fig1) is relatively sheltered from heavy wave action It was visited numeroustimes in January 1969 and from January to March 1970 The sub-stratum consists of an eroded reef-limestone bench merging with a beachof coarse calcareous sand at the shoreward edge The rocks in the littoralfringe and higher intertidal zone are sand-scoured during storms whichoccur mainly in winter The sublittoral fringe supports a luxuriant growthof algae including Acanthophora Halimeda Sargassum Padina andValonia as well as encrusting corals including Porites astreoides Siderastrearadians Isophyllia sinuosa and Millepora In somewhat deeper waterisolated heads of Diploria and Acropora palmata may be found Severalechinoids (Echinometra lucunter Diadema antillarum Tripneustes ven-tricosus Lytechinus variegatus and Eucidaris tribuloides) abound in self-made cavities Few molluscs inhabit open rocky surfaces in the sublittoralfringe the most commonly encountered gastropods are Leucozonia leuco-zonalis Cerithium litteratum Cymatium nicobaricum and Fissurella bar-badensis while Astraea tecta and Ocenebra intermedia are less commonThe fauna of crevices and the undersurfaces of stones is large and variedincluding several gastropods (see Table 3) and the bivalves Arcopsisadamsi Arca zebra Barbatia sp Chama sp lsognomon radiatus and

1973] Vermeij West Indian Molluscan Communities 357occasional distorted Modiolus americanus A shallow bed of Thalassiaadjacent to and somewhat overlapping this locality was not included inthe morphological analysis

The littoral fringe supports very few animals small specimens ofTectarius muricatus Nodilittorina tuberculata Littorina lineata L lineolataand Nerita versicolor are occasional In the high intertidal zone largeindividuals of Nerita tessellata and N fulgurans are quite common bothon open surfaces and beneath stones Crevices in the intertidal oftensupport mats of the small bivalves Isognomon bicolor and Brachidontesdominguensis interspersed with the larger I radiatus and Pinctada radiataThe last species are more characteristic of subtidal surfaces

Locality 3 Boca Playa Canoa Curafao-This locality on the heavily wave-exposed north coast of Curaltaowas visited in September 1969 and againin April 1970 The shore consists of a very wide honeycombed limestoneterrace lying in the littoral fringe which at its seaward edge is succeededin stepwise fashion by two lower platforms The zonation pattern on thehighest terrace is similar to that at Fort Point except that Echininus isabsent Flatbottomed pools in the littoral fringe contain Littorina lineataL minima Nodilittorina tuberculata Planaxis nucleus and P lineatusThe zone of mosslike algae on vertical surfaces at the seaward edge ofthe highest platform supports the chiton Acanthopleura granulata thegastropods Acmaea leucopleura and Purpura patula and the crevice-dwelling bivalves Isognomon bicolor and Brachidontes dominguensis Lit-torina meleagris was not found N erita tesselata is abundant on the middleplatform together with P patula Thais rustica Cittarium pica Acmaeajamaicensis Fissurella nodosa F barbadensis and Acanthopleura granu-lata These species with the exception of Purpura and N tessellata con-tinue to the lowest platform where they are accompanied by Leucozoniaocellata and a few individuals of Conus mus The echinoids Echinometralucunter and Lytechinus variegatus are abundant on the two lower plat-forms A luxuriant algal growth including large plants of Sargassumoccurs on the lowest platform where occasional small encrusting coloniesof Porites astreoides are also encountered

Locality 4 Piscadera Baai Curafao-This locality lying in front of theCaraibisch Marien-Biologisch Instituut about 4 km northwest of Willemstadon the south coast of Curaltaowas visited in January 1968 September1969 and April 1970 The shore is a relatively sheltered beach of erodedcoral rubble Molluscs are absent from most of the littoral fringe exceptin its lowest portion from the mean high-water mark to about 30 emabove which is inhabited by Nerita versicolor Planaxis nucleus andLittorina lineolata Open surfaces below the water line support Acmaeapustulata N erita tessellata and the xancid Vasum capitellus The echinoids

358 Bulletin of Marine Science [23(2)

Diadema antillarum and Echinometra lucunter and the encrusting coralSiderastrea radians are abundant but there is no luxuriant growth ofmacroscopic algae The fauna of crevices and the undersurfaces of stonesis quite rich supporting several gastropods (see Table 5) and the bivalvesArcopsis adamsi Barbatia sp and Isognomon radiatus

Locality 5 Vieux Habitants Guadeloupe-This locality visited in Feb-ruary 1971 lies on the west coast of the island of Guadeloupe about 9km north of Basse Terre and 2 km south of Vieux Habitants near theHotel Rocroy The area investigated consists of a V-shaped cove oneither side of which are outcrops of exfoliating volcanic rock separated fromone another by a beach of fine black sand Both sides of the cove areexposed to rather heavy wave action The north side of the cove (Locality5A) is severely sand-scoured and supports a somewhat unusual epifaunaThe highest shore levels are occupied by small eroded individuals ofNodilittorina tuberculata Littorina lineata L ziczac and Nerita versicolorThe rocks in the high intertidal zone support l-cm2 patches of mosslikealgae among which occur Littorina meleagris and an apparently undescribedlimpet of the genus Acmaea whose shell has a smooth exterior radiallyornamented with black and white stripes At middle and lower intertidallevels on open surfaces are found Fissurella nimbosa Acmaea sp indetand Chiton marmoratus the last-named species being generally somewhatsheltered from the most severe wave action Crevices in the rocks at lowlevels contain rather smooth individuals of Thais rustica and the bivalveslsognomon bicolor and Brachidontes dominguensis Small growths ofSargassum occupy the most seaward parts of the outcrops

The south side of the cove (Locality 5B) is not subjected to sandabrasion From about 100 cm to 30 em above the upper limit of a zoneof mosslike algae are found small individuals of Tectarius muricatusNodilittorina tuberculata and Littorina lineata From the upper limitof mosslike algae to 30 cm above may be found L ziczac L lineolataNerita versicolor and N peloronta The uppermost reaches of the moss-like algal zone contain on open and semiprotected surfaces the gastropodsNerita tessellata Acmaea leucopleura L ziczac and Planaxis nucleusAn occasional individual of the undescribed Acmaea may also be foundAt lower levels occur the chitons Acanthopleura granulata and Chiton mar-moratus and the gastropods N tessellata Acmaea antillarum Fissurellanodosa F barbadensis Tegula excavata Cittarium pica Purpura patuaand strongly noded individuals of Thais rustica In contrast to the northside of the cove the echinoid Echinometra lucunter and the large barnacleTetraclita stalactifera are abundant at low intertidal levels on the southside and encrusting colonies of the fire coral Millepora occur in the sub-littoral fringe

1973] Vermeij West Indian Molluscan Communities 359

c

B

AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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364 Bulletin of Marine Science [23(2)

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366 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 357occasional distorted Modiolus americanus A shallow bed of Thalassiaadjacent to and somewhat overlapping this locality was not included inthe morphological analysis

The littoral fringe supports very few animals small specimens ofTectarius muricatus Nodilittorina tuberculata Littorina lineata L lineolataand Nerita versicolor are occasional In the high intertidal zone largeindividuals of Nerita tessellata and N fulgurans are quite common bothon open surfaces and beneath stones Crevices in the intertidal oftensupport mats of the small bivalves Isognomon bicolor and Brachidontesdominguensis interspersed with the larger I radiatus and Pinctada radiataThe last species are more characteristic of subtidal surfaces

Locality 3 Boca Playa Canoa Curafao-This locality on the heavily wave-exposed north coast of Curaltaowas visited in September 1969 and againin April 1970 The shore consists of a very wide honeycombed limestoneterrace lying in the littoral fringe which at its seaward edge is succeededin stepwise fashion by two lower platforms The zonation pattern on thehighest terrace is similar to that at Fort Point except that Echininus isabsent Flatbottomed pools in the littoral fringe contain Littorina lineataL minima Nodilittorina tuberculata Planaxis nucleus and P lineatusThe zone of mosslike algae on vertical surfaces at the seaward edge ofthe highest platform supports the chiton Acanthopleura granulata thegastropods Acmaea leucopleura and Purpura patula and the crevice-dwelling bivalves Isognomon bicolor and Brachidontes dominguensis Lit-torina meleagris was not found N erita tesselata is abundant on the middleplatform together with P patula Thais rustica Cittarium pica Acmaeajamaicensis Fissurella nodosa F barbadensis and Acanthopleura granu-lata These species with the exception of Purpura and N tessellata con-tinue to the lowest platform where they are accompanied by Leucozoniaocellata and a few individuals of Conus mus The echinoids Echinometralucunter and Lytechinus variegatus are abundant on the two lower plat-forms A luxuriant algal growth including large plants of Sargassumoccurs on the lowest platform where occasional small encrusting coloniesof Porites astreoides are also encountered

Locality 4 Piscadera Baai Curafao-This locality lying in front of theCaraibisch Marien-Biologisch Instituut about 4 km northwest of Willemstadon the south coast of Curaltaowas visited in January 1968 September1969 and April 1970 The shore is a relatively sheltered beach of erodedcoral rubble Molluscs are absent from most of the littoral fringe exceptin its lowest portion from the mean high-water mark to about 30 emabove which is inhabited by Nerita versicolor Planaxis nucleus andLittorina lineolata Open surfaces below the water line support Acmaeapustulata N erita tessellata and the xancid Vasum capitellus The echinoids

358 Bulletin of Marine Science [23(2)

Diadema antillarum and Echinometra lucunter and the encrusting coralSiderastrea radians are abundant but there is no luxuriant growth ofmacroscopic algae The fauna of crevices and the undersurfaces of stonesis quite rich supporting several gastropods (see Table 5) and the bivalvesArcopsis adamsi Barbatia sp and Isognomon radiatus

Locality 5 Vieux Habitants Guadeloupe-This locality visited in Feb-ruary 1971 lies on the west coast of the island of Guadeloupe about 9km north of Basse Terre and 2 km south of Vieux Habitants near theHotel Rocroy The area investigated consists of a V-shaped cove oneither side of which are outcrops of exfoliating volcanic rock separated fromone another by a beach of fine black sand Both sides of the cove areexposed to rather heavy wave action The north side of the cove (Locality5A) is severely sand-scoured and supports a somewhat unusual epifaunaThe highest shore levels are occupied by small eroded individuals ofNodilittorina tuberculata Littorina lineata L ziczac and Nerita versicolorThe rocks in the high intertidal zone support l-cm2 patches of mosslikealgae among which occur Littorina meleagris and an apparently undescribedlimpet of the genus Acmaea whose shell has a smooth exterior radiallyornamented with black and white stripes At middle and lower intertidallevels on open surfaces are found Fissurella nimbosa Acmaea sp indetand Chiton marmoratus the last-named species being generally somewhatsheltered from the most severe wave action Crevices in the rocks at lowlevels contain rather smooth individuals of Thais rustica and the bivalveslsognomon bicolor and Brachidontes dominguensis Small growths ofSargassum occupy the most seaward parts of the outcrops

The south side of the cove (Locality 5B) is not subjected to sandabrasion From about 100 cm to 30 em above the upper limit of a zoneof mosslike algae are found small individuals of Tectarius muricatusNodilittorina tuberculata and Littorina lineata From the upper limitof mosslike algae to 30 cm above may be found L ziczac L lineolataNerita versicolor and N peloronta The uppermost reaches of the moss-like algal zone contain on open and semiprotected surfaces the gastropodsNerita tessellata Acmaea leucopleura L ziczac and Planaxis nucleusAn occasional individual of the undescribed Acmaea may also be foundAt lower levels occur the chitons Acanthopleura granulata and Chiton mar-moratus and the gastropods N tessellata Acmaea antillarum Fissurellanodosa F barbadensis Tegula excavata Cittarium pica Purpura patuaand strongly noded individuals of Thais rustica In contrast to the northside of the cove the echinoid Echinometra lucunter and the large barnacleTetraclita stalactifera are abundant at low intertidal levels on the southside and encrusting colonies of the fire coral Millepora occur in the sub-littoral fringe

1973] Vermeij West Indian Molluscan Communities 359

c

B

AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

ooN

o00

ooN

ot-- o

00- o00ltt

o00

o-oVl

-

oVl0

0Vl00o

01obullbull

0oN

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364 Bulletin of Marine Science [23(2)

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366 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

358 Bulletin of Marine Science [23(2)

Diadema antillarum and Echinometra lucunter and the encrusting coralSiderastrea radians are abundant but there is no luxuriant growth ofmacroscopic algae The fauna of crevices and the undersurfaces of stonesis quite rich supporting several gastropods (see Table 5) and the bivalvesArcopsis adamsi Barbatia sp and Isognomon radiatus

Locality 5 Vieux Habitants Guadeloupe-This locality visited in Feb-ruary 1971 lies on the west coast of the island of Guadeloupe about 9km north of Basse Terre and 2 km south of Vieux Habitants near theHotel Rocroy The area investigated consists of a V-shaped cove oneither side of which are outcrops of exfoliating volcanic rock separated fromone another by a beach of fine black sand Both sides of the cove areexposed to rather heavy wave action The north side of the cove (Locality5A) is severely sand-scoured and supports a somewhat unusual epifaunaThe highest shore levels are occupied by small eroded individuals ofNodilittorina tuberculata Littorina lineata L ziczac and Nerita versicolorThe rocks in the high intertidal zone support l-cm2 patches of mosslikealgae among which occur Littorina meleagris and an apparently undescribedlimpet of the genus Acmaea whose shell has a smooth exterior radiallyornamented with black and white stripes At middle and lower intertidallevels on open surfaces are found Fissurella nimbosa Acmaea sp indetand Chiton marmoratus the last-named species being generally somewhatsheltered from the most severe wave action Crevices in the rocks at lowlevels contain rather smooth individuals of Thais rustica and the bivalveslsognomon bicolor and Brachidontes dominguensis Small growths ofSargassum occupy the most seaward parts of the outcrops

The south side of the cove (Locality 5B) is not subjected to sandabrasion From about 100 cm to 30 em above the upper limit of a zoneof mosslike algae are found small individuals of Tectarius muricatusNodilittorina tuberculata and Littorina lineata From the upper limitof mosslike algae to 30 cm above may be found L ziczac L lineolataNerita versicolor and N peloronta The uppermost reaches of the moss-like algal zone contain on open and semiprotected surfaces the gastropodsNerita tessellata Acmaea leucopleura L ziczac and Planaxis nucleusAn occasional individual of the undescribed Acmaea may also be foundAt lower levels occur the chitons Acanthopleura granulata and Chiton mar-moratus and the gastropods N tessellata Acmaea antillarum Fissurellanodosa F barbadensis Tegula excavata Cittarium pica Purpura patuaand strongly noded individuals of Thais rustica In contrast to the northside of the cove the echinoid Echinometra lucunter and the large barnacleTetraclita stalactifera are abundant at low intertidal levels on the southside and encrusting colonies of the fire coral Millepora occur in the sub-littoral fringe

1973] Vermeij West Indian Molluscan Communities 359

c

B

AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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364 Bulletin of Marine Science [23(2)

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366 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 373

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374 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 375

tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 359

c

B

AFIGURE 2 Measurements taken on coiled gastropod shells Point A is theapex of the shell at the origin of a cylindrical coordinate system The axisof coiling which bisects angle A is coincident with the positive part of theZ-axis

METHODS

Geometrical Analysis-The basically logarithmic growth pattern of the ac-cretionary molluscan shell reduces the number of parameters for an ade-quate mathematical description to a relatively small number For thepresent analysis four geometrical parameters developed by Raup (19661967) and Vermeij (1971a) have been chosen they are defined as follows

Consider a dextral orthostrophic conispirally coiled shell with theapex A at the origin of an r () z cylindrical coordinate system with theaxis of coiling on the positive part of the Z-axis (see Fig 2) Consider apoint B on the outer lip of the shell whose distance r from the axis ofltoiling is greater than that of any other point on the outer lip Consideranother point C corresponding to B but one-half revolution back fromthe outer lip Let d be the distance between Band C then the distancefrom C to the axis of coiling is d - r The expansion rate W of thewhorls is then defined as W = [r (d - r)]2

The translation rate T of point B along the Z-axis is defined as

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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364 Bulletin of Marine Science [23(2)

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366 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 373

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

360 Bulletin of Marine Science

POINTER~ PROTRACT7

STAND

[23(2)

FIGURE 3 Schematic diagram of the reflecting goniometer used in obtainingangles of gravitational stability

T = cot (AI2) where AI2 is the apical half-angle of the shell The shapeS of the generating curve or aperture is defined to be the ratio S = ljwwhere l is the greatest linear dimension of the generating curve and wis the maximum width of the aperture measured perpendicular to 1 Finallythe angle of elevation E of the axis of coiling above the plane of thegenerating curve is defined as cot E = ozo(J (Vermeij 1971a)

The four parameters above (W T E and S) were calculated from thefollowing measurements made on each coiled gastropod shell ( 1) thedistance C between A and B (see Fig 2) (2) the distance b betweenA and C (3) the distance d between Band C (4) the angle E and(5) the lengths land w

The axis of coiling bisects angle CAB and divides line BC into segments

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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364 Bulletin of Marine Science [23(2)

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366 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 361

of lengths rand d - r From the law of cosines cos A = (d2 - c2 - b2)1-2cb From the law of sines r = cmiddotsin (AI2) and W = ([cmiddot sin (AI2)] d - [c sin (A 12) ] )2 It should be pointed out that T and Ware not inde-pendent of one another As W increases T decreases in most cases andVIce versa

In addition to the four geometrical parameters derived above twomeasures of gravitational stability were made using a reflecting goniometer(Fig 3) A shell is placed with the aperture facing down on a horizontalplane surface of sandpaper If in this position the shell remains stationarywith the aperture facing down it is said to be gravitationally stable Todetermine the extent of gravitational stability the horizontal surface uponwhich the shell rests may be tilted to an angle beyond which the shellwill no longer remain with the aperture facing down on the surfaceThe two angles of gravitational stability measured for each empty coiledgastropod shell are (1) the posterior stability angle from the horizontalG7 needed to destabilize the shell so that it falls backwards towards itsapex and (2) the lateral stability angle from the horizontal Gw neededto destabilize the shell in a direction opposite to the outer lip of theaperture The greater the angles of stability the greater must be thedeviation from the horizontal to upset the shells position with the aperturefacing down and hence the greater the stability of the shell The anglesof stability depend on the logarithmic and sculptural properties of the shellthe density of the shell relative to that of the animal as a whole and theextent to which the margins of the aperture conform to the rocky surfaceduring life While measuring the gravitational characteristics of the emptyshell need not necessarily inform us about the gravitational properties ofthe animal as a whole the angles of stability as defined here neverthelessconstitute a convenient basis for comparing gravitational characteristicsof species with different shapes and masses It must in any case be keptin mind that the gravitational characteristics of the whole animal willchange whenever the soft parts extend from or withdraw into the shellFurther considerations of the relation between gravitational properties andshell form in gastropods will be taken up in a subsequent paper

In practice it was somewhat difficult to measure certain of the geo-metrical and gravitational parameters accurately This is particularly truewhen the apex is eroded as in many individuals of Nerita Thais andNitidella or when the shell is heavily encrusted by calcareous Litho-thamnium

Acmaeid and fissurellid limpets while still following a logarithmic growthpattern have reverted to planispiral coiling (T = 0) and have essentiallylost internal spiral structure as a result of the very high rate of whorlexpansion W The limpet shell is essentially a cone with the apexor fissure at the top and an ovate generating curve at the base The

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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364 Bulletin of Marine Science [23(2)

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366 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 373

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

362 Bulletin of Marine Science [23(2)

following measurements were taken on each limpet shell (1) the majordiameter I of the base (2) the minor diameter w of the base (3)height h of the apex above the base (4) distance a from the apex tothe anterior margin of the shell and (5) distance b from the apex tothe posterior margin of the shell Three ratios were then calculated (1 )shape S of the base S = wl (2) relative height hh of the shellhh=h(lw) and (3) eccentricity X of the apex X=ab Nogravitational stability measurements were attempted on limpets

Biogeographic Analysis-Data on biogeographic distribution were obtainedfrom Russell (1941) Bequaert (1943) Farfante (1943) Nickles (1950)Abbott (1954) Warmke amp Abbott (1962) Matthews amp Rios (1967)Work (1959) Vermeij amp Porter (1971) and Vermeij (1972) Specieswere considered as belonging to one of the following categories ( 1)those restricted to the Caribbean region ie not extending south andeast of Trinidad and Barbados and not north of southeast Florida (2)those extending past the mouth of the Amazon to east Brazilian watersand (3) those occurring either on both sides of the Atlantic or in thewestern Atlantic and Pacific Although most members of category (3)also belong to category (2) this need not be the case for example Ihave collected specimens indistinguishable from West Indian individualsof Littorina meleagris at Takoradi Ghana but there is no indication atpresent that this species occurs in Brazil Since natural rocky shores arealmost non-existent north of the Florida Keys along the east coast ofthe United States it is felt that an analysis of geographic distribution asa function of habitat based on whether the species is limited to thetropics or extends into temperate waters (see Vermeij 1972) would yieldlittle relevant information

RESULTS

Morphology-Tables 2-6 present morphological and distributional datafor the gastropods at the five localities described above One of the mostapparent of the several trends in the data is that species on open surfacesat low shore levels tend to have high A2 and W values and relatively highangles of stability At localities 1 3 and 5 which are exposed to ratherheavy wave action open-surface species of the lower shore tend to haveA2 values greater than 25deg and W values greater than 135 The onlyexception to this statement is constituted by Conus regius at Loc 1 inwhich W = 125 Posterior and lateral stability angles never fall below20deg and 12 respectively in this group and for most species are generallymuch greater than this In all but one case the posterior stability angleGT is greater than the lateral stability angle GV the exceptional species(Tegula excavata at Loc 5) has very high stability the two anglesbeing more or less equal In the more sheltered Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 363

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 363

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

364 Bulletin of Marine Science [23(2)

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surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

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the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973J Vermeij West Indian Molluscan Communities 365o000o

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366 Bulletin of Marine Science [23(2)

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surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

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the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

366 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

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the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 373

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 367

surface-dwelling species at low shore levels may have AI2 valuesas low as 19deg and W values as low as 110 (Cerithium litteratumLoc 2) Stability angles are generally high but C litteratum is gravita-tionally unstable both laterally and posteriorly Other posterior angles aslow as 8deg (Vasum capitellus Loc 4) and lateral angles as low as 14deg(Coralliophila abbreviata) also occur Most surface-dwellers in low-shoresheltered habitats have greater posterior than lateral stability Vasumcapitellus (Loc 4) is one exception

Gastropods living on open surfaces in the littoral fringe are morpho-logically like their lower-shore counterparts but the range of the severalparameters is somewhat greater than for low-shore species on wave-exposedshores The apical half-angle A12 and expansion rate W may be aslow as 22 deg and 123 respectively (Littorina lineata Locs 1 and 3) whileat the other extreme they may be as high as 375deg and 247 (Neritapeloronta Locs 1 and 3) Posterior stability angles vary from 65deg(N peloronta) to 10deg (L ziczac Loc 3) while lateral stability liesbetween 48deg (N peloronta) and 14deg (L ziczac) Although lateral stabilityis generally equal to or less than posterior stability several species in-cluding L ziczac and N peloronta (large individuals) exhibit the reverse

The gastropod fauna of crevices and the undersurfaces of stones ismorphologically very diverse ranging from such high-spired forms asMitra nodulosa at Loc 2 (A12 = 13deg W = 109) to such low-spiredspecies as Tegula jasciata at Loc 2 (A12 = 37deg W= 162) Althoughthere is a considerable overlap with the morphological range of surface-dwelling species expansion rate (W) in particular does not attain thehigh values which it does in the latter group Posterior stability anglesvary from horizontally unstable in Cerithium eburneum at Loc 2 to 58degin Tegula hotessieriana at Loc 4 but most species fall well below thislast extreme only one other (T fasciata) exceeding 44deg Similarly whilethe highest lateral stability angle is 52deg (T jasciata at Loc 2) all butone other species (T hotessieriana) have lateral stabilities below 25degOf 14 species living cryptically at Locs 2 4 and 5 seven have greaterlateral than posterior stability and seven have greater posterior than lateralstability

Inhabitants of littoral-fringe splashpools are similar to the cryptic low-shore fauna in their morphological diversity ranging from the high-spiredBatillaria minima at Loc 1 (A12 = 15deg W = 110) to the globose Puperitapupa at Loc 1 (A12 = 35deg W = 201) Posterior stability varies fromhorizontally unstable (Batillaria) to G7 = 44deg (Puperita) Lateral stabilityranges between horizontally unstable (Batillaria) and Gw = 24deg (Puperitaand Littorina mespillum at Loc 1) Posterior stability is greater thanlateral stability in all species except Planaxis lineatus (Loc 3)

The angle of elevation (E) of the axis of coiling above the plane of

368 Bulletin of Marine Science [23(2)oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 369oClCl

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370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

370 Bulletin of Marine Science [23(2)

the aperture varies widely in all ecological groups Forms with E greaterthan 45deg are found among surface-dwellers of the upper and lower shoreand among those living beneath stones they are lacking in the limitedlittoral-fringe splashpool fauna Species with E less than 30deg are commonin all habitats

The shape of the aperture is relatively uniform among surface-dwellinggastropods of the littoral fringe varying from 0737 (Nerita versicolorLoc 1) to 136 (Littorina lineolata Loc 1) and always being ovate toround The same applies to species living in splashpools at high shorelevels Among low-shore open-surface species apertural form varies fromvery elongate (Conus Locs 1 and 3) to transversely ovate (Astraea andTegula Locs 1 and 5 respectively) All species with S greater than200 possess longitudinally ovate apertures which are extended anteriorlyby a narrow siphonal canal The two species of Conus (C regius at Loc1 S = 719 and C mus at Loc 3 S = 621) tend to be associated withlocally somewhat sheltered hollows or pockets of sand and the apertureis of approximately equal width along its entire length Gastropods livingbeneath boulders are characterized by a great diversity of apertural formsranging from 0895 in Tegula fasciata at Loc 2 to 276 in Mitra nodulosaat the same locality In these forms and in low-shore species living onsheltered open surfaces apertures whose length is greater than twicethe width are often rectangular or ellipsoid without an extended anteriorsiphonal canal

Size of species is variable in most habitats Among surface-dwellinggastropods of the littoral fringe at Loc 1 (Fort Point Jamaica) thesmallest species (Littorina lineata) reaches a maximum linear dimensionof 130 mm while the largest species (Nerita peloronta) reaches a size of391 mm In the high intertidal surface-dwellers range from the smallL meleagris (maximum size 59 mm) to the large Purpura patula (maxi-mum size 660 mm) Low-shore surface-dwelling gastropods range fromthe small Fissurella barbouri (maximum major diameter 93 mm) to therelatively large Astraea caelata (maximum diameter 460 mm) Subtidalspecimens of the surface-dwelling Cittarium pica at this locality may reacha diameter of 100 mm At Lac 2 (Discovery Bay) maximum size amongcryptic gastropods ranges from 100 mm (Hyalina avena) to 328 mm(Mitra nodulosa) with most species not exceeding 25 mm maximumsize among surface-dwelling species ranges from 110 mm (Ocenebraintermedia) to 660 mm (Cymatium nicobaricum) with all but onespecies exceeding 20 mm At Loc 4 (Piscadera Baai Curayao) maxi-mum size among cryptic species varies from 96 mm (Nitidella ocellata)to 250 mm (Cantharus lautus) and among open-surface forms from165 mm (Nerita tessellata) to 549 mm (Vasum capitellus) It is verylikely that many small species were overlooked at both Locs 2 and 4

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 371

because of the ease with which they fall off the undersurfaces of stonesSpecies of littoral-fringe splashpools at Locs 1 and 3 are uniformly smallonly one species (Acmaea pustulata at Loc 1 204 mm) exceeds 13mm in maximum linear dimension While open-surface species at lowlevels at Locs 3 and 5 appear to be generally small most belong tospecies which become much larger in other localities Small size at Loc5 may be related to the presence of volcanic substratum

The general trends outlined in the preceding paragraphs may be in-vestigated in greater detail by examining particular supraspecific taxaIn the trochid genus Tegula the species which is gravitationally moststable and which attains the greatest size is the open-surface-dwelIingTegula excavata (Loc 5) Although its expansion rate (W = 150) issmaller than that of the other two species encountered in this study bothof which live beneath stones its angle of elevation (E) of the coiling axisabove the plane of the aperture is higher (65deg) The gravitational stabilitycharacteristics and external appearance of T excavata are very nearlylike those of a right circular cone with the broad aperture occupyingnearly the entire basal area The other two species (T fasciata at Loc2 and T hotessieriana at Loc 4) have the apex located distinctly towardthe posterior end of the shell and the aperture is relatively smallerThe turbinid genus Astraea is convergent to Tegula Of the two speciesencountered in this study the larger A caelata has higher A2 W andGlI values is more strongly ornamented with peripheral knobs and livesin more wave-exposed habitats than A tecta The difference between pos-terior and lateral stability is somewhat greater in A tecta (G7 - Gil = 9deg)than in A caelata (4deg)

Two species of the genus Planaxis are known from the West Atlanticand both occur in the localities studied The larger lower-spired Pnucleus occurs on open surfaces at high intertidal levels on shelteredboulder beaches or in high-shore splashpools on more wave-exposed shoresThe small highly gregarious P lineatus occurs always submerged in waterbeneath stones or in high-shore splashpools Its more cryptic habits arereflected by the more slender profile of the shell (lower A2 and Wvalues) and lower posterior and lateral stability than in P nucleus

Morphological trends in littorinids neritids and acmaeid limpets willbe treated more fully elsewhere (Vermeij in press) The ratio of heightof the spire to length of the aperture and the degree of development ofexternal shell ornamentation are generally greater among littorinids ofthe littoral fringe than among those of splashpools or open high inter-tidal surfaces Among neritids species in the lower littoral fringe aremore globose than are those on mid- to high-intertidal surfaces Acmaeidlimpets are represented by rather flat species at low shore levels and bymore high-conical species at high shore levels There is also a distinct

372 Bulletin of Marine Science [23(2)

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

372 Bulletin of Marine Science [23(2)

oN

oNbullbull

bull

ooN

olttbull

o01

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01bullbull

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 373

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374 Bulletin of Marine Science [23(2)

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tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

374 Bulletin of Marine Science [23(2)

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1973] Vermeij West Indian Molluscan Communities 375

tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 375

tendency for species of the upper shore (Acmaea sp indet A leucopleura)to be smaller than their lower-shore congeners (A antillarlm A ja-maicensis A pustulata) judging from maximum major diameter

The West Indian lower-shore keyhole limpets (Fissurellidae) are mor-phologically like acmaeid limpets in the same habitat except that a fissureoccupies the apical region of the shell and that they are often relativelymore high-conical F nodosa the most strongly sculptured species isrestricted to open surfaces on heavily wave-exposed shores The next mostsculptured species F barbadensis occurs on both exposed and shelteredshores (see also Ward 1967) The large relatively flat F nimbosa isnearly smooth externally and was found only on open surfaces at LocSA where it was subjected to severe sand scouring It is interesting thatthe other limpet restricted to open sand-scoured surfaces Acmaea spindet also lacks external shell ornamentation and that specimens ofThais rustica from sand-scoured habitats are much smoother than thosefrom adjacent sand-free surfaces at Loc SB Strongly ornamented speciessuch as Fissurella nodosa and Purpura patula could not be found onthe sand-scoured surfaces but they were abundant in the adjacent sand-free area These observations agree well with those made on the inter-tidal molluscan fauna of sand-scoured open surfaces in PernambucoBrazil and southwest Ecuador (Vermeij amp Porter 1971)

Biogeography-A summary of distributional data is given in Table 7From these data it is readily apparent that all littoral-fringe species whetherthey live on open surfaces or in pools are restricted to the CaribbeanProvince (Category 1) Indeed the three spinose littorinids Echininusnodulosus Nodilittorina tuberculata and Tectarius muricatus all belong tomonotypic subgenera (Rosewater 1970 and pers commun) which aretherefore also restricted to the Caribbean N erita peloronta is also theonly member of the subgenus Nerita s str

About 20-30 per cent of low- to high-intertidal and open-surface specieson shores exposed to heavy wave action (Locs 1 3 and 5) and onsome shores with less wave action (Lac 4) include Brazil in their geo-graphic ranges and one species (Littorina meleagris) is trans-Atlanticthe rest are restricted to the Caribbean Province At the more shelteredLocality 2 two-thirds of the lower-shore open-surface species of gastropodsare found in Brazil and the Caribbean and two species are transoceanicBarring one unidentified turrid at both Locs 2 and 4 all cryptic gastropodsat these localities occur both in Brazil and in the Caribbean At Loc 2one species out of eight living beneath stones is transoceanic while at Loc4 three out of nine are transoceanic

From these results it is clear that gastropods of the upper shore aresignificantly narrower in their geographic range than either cryptic speciesor open-surface species of the lower shore In addition it is apparent that

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

376 Bulletin of Marine Science [23(2)

TABLE 7SUMMARY OF GEOGRAPffiC DISTRIBUTION OF GASTROPODS AS A FUNCTION

OF HABITAT

Species belonging toNumber Category 1 Category 2 Category 3

ofspecies (No) () (No) () (No) ()

Fort Point Jamaica (Lac 1)low intertidal open surfaces 9 7 78 2 22 0 0high intertidal open surfaces 4 2 50 1 25 1 25littoral fringe splashpools 5 5 100 0 0 0 0littoral fringe open surfaces 8 8 100 0 0 0 0

Discovery Bay Jamaica (Lac 2)low intertidal open surfaces 6 2 33 4 67 2 33low intertidal cryptic 9 0 0 8 100 I 11

Boca Playa Canoa Cura9ao(Lac 3)

littoral fringe open surfaces 7 7 100 0 0 0 0littoral fringe splashpools 3 3 100 0 0 0 0low intertidal open surfaces 7 5 71 2 29 0 0high intertidal open surfaces 3 2 67 I 33 0 0

Piscadera Baai Cura9ao (Lac 4)low intertidal open surfaces 6 4 67 2 33 0 0low intertidal cryptic 9 0 0 8 100 3 33

Vieux Habitants Guadeloupe(Lac 5)

low intertidal open surface 12 10 83 2 17 0 0Category 1 restricted to the Caribbean Category 2 extending to Brazil Category 3 transoceanic

among low-shore species those living on open wave-exposed surfaces havenarrower geographic ranges than those living on sheltered open surfaces orbeneath stones These results confirm those obtained for the molluscs inPernambuco (Vermeij amp Porter 1971) and other areas in the tropical SouthAtlantic (Vermeij 1972)

It is interesting to note that the two open-surface chitons encounteredin this study (Acanthopleura granulata and Chiton marmoratus) are bothrestricted to the Caribbean Province Furthermore the two limpets re-stricted at Loc 5 to sand-scoured surfaces both have a limited rangewithin the Caribbean The keyhole limpet Fissurella nimbosa is restrictedto the southern Caribbean as it does not occur north of Puerto Rico(Farfante 1943) The undescribed Acmaea is represented in the collec-tions at the American Museum of Natural History New York by onlytwo lots one from Martinique and the other from Venezuela and islikely also restricted to the southern Caribbean

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 377

TABLE 8SUMMARY OF MOLLUSCAN MORPHOLOGY IN DIFFERENT CARIBBEAN

ROCKy-SHORE HABITATS

Parameter

SizeAI2WSG7

Gw

Littoralfringe

130 to 391 mm22deg00 to 37deg30123 to 247073 to 13610deg to 65deg14degt058deg

Low-shoreopen-surfacewave-ex posed

59 to 660 mm25deg00 to 39deg00135 to 261070 to 22520deg to 62deg12deg to 62deg

Low-shorecryptic

100 to 322 mm13deg00 to 37deg00109 to 162090 to 276unstable to 58degunstable to 52deg

DISCUSSION

Gastropods living on open surfaces at low levels of the shore inwave-exposed areas have rather uniformly high A12 W Gr and Gilvalues and apertures tend to be ovate to round in form They may varygreatly in size but many species tend to be relatively large Open-surfacespecies in more sheltered low-shore rocky habitats may take on lowerA12 W Gr and Gw values than their more wave-exposed counterpartsand the aperture may be quite elongate Littoral-fringe species on opensurfaces may also have A12 W Gr and Gw values somewhat lower thanthose of wave-exposed low-shore species but the aperture is uniformlyovate to round Species in this category range somewhat less widely insize than at lower shore levels High-level splashpools contain a limitedfauna of small species with ovate apertures and widely ranging A12 W G7middot

and GlI values Among lower-shore species living cryptically in crevicesor beneath stones there is a great diversity in all parameters except overallsize which tends to be small These general trends suggest that wheresize diversity is greatest morphological diversity as defined by the rangeof A12 W S C7 and Cw is least Alternatively where size diversity isleast morphological diversity appears to be greatest

The relative morphological uniformity of the gastropod assemblage onopen surfaces at low levels on wave-exposed shores may reflect the es-sentially similar way in which taxonomically unrelated groups have adaptedto the same physical regime The tendency towards high expansion ratelow translation rate (high AI2) and ovate to round apertures reflectsgenerally high posterior and lateral gravitational stability and is perfectedin the many limpets that occur in this assemblage In some coiled gastro-pods such as Astraea caelata and Tegula excavata and in many limpetsthere is a tendency towards the development of a secondary radial sym-metry These morphological trends appear to be mostly adaptations togreat water turbulence It should be pointed out however that gravita-

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

378 Bulletin of Marine Science [23(2)

tional stability and hence presumably adaptations to water turbulencecan be achieved among gastropods by strategies other than increasing AI2and W This is particularly well illustrated by the open-surface-dwellingCymatium nicobaricum at Loc 2 which derives its high posterior andlateral stability not from low translation rate and high expansion ratebut from the presence of large varices and other sculptural elements onthe shell parallel to the outer lip and on the shoulders of the whorls

While stability requirements are somewhat relaxed on open surfacesin the littoral fringe as reflected by the greater range of A12 W GT andOw at the lower end of the scale the often conflicting requirements ofavoiding desiccation and maintaining a favorable body temperature mayimpose limitations on the shape of the aperture Gastropods have evolvedtwo morphologically and adaptively distinct types to deal with the rigorsof temperature and desiccation at high shore levels (Vermeij 1971c andin press) The neritids which remain attached to the substratum whenexposed to the air probably employ evaporative cooling to lower theirbody temperature relative to that of the external environment (Lewis1963 Vermeij 1971b) They must seek a compromise among minimizingdesiccation minimizing heat conduction through contact with the sub-stratum retaining a sufficiently large reservoir of water and evaporatingenough water for effective cooling At the same time this mode of liferequires a considerable degree of gravitational stability so that attachmentto the substratum may be assured An increase in shell globosity generallyimplies a smaller surface in contact with the substratum a smaller surfacefrom which water can evaporate a larger water reservoir and reducedgravitational stability (Vermeij in prep) The reduction in gravitationalstability by the more globose upper-shore species relative to the flatterlower-shore species can be tolerated by Nerita versicolor because of re-duced turbulence and is counteracted by Nerita peloronta which oftenlives on exposed vertical surfaces by greater clinging power and by agreater conformity of the apertural margins with the substratum

The littorinids the second morphological and taxonomic category ofopen-surface gastropods in the littoral fringe withdraw into their shellswhen exposed to the air and attach themselves to the substratum onlyby means of a dried film of mucus at the outer lip of the shell They donot rely on evaporative cooling and therefore need neither a large reservoirof water nor a surface of tissue from which evaporation takes placeApertural size should be as small as possible so as to minimize water lossand heat conduction from the substratum With these adaptations lit-torinids can and do live at higher shore levels than neritids Their re-quirements for gravitational stability would be generally less than those ofneritids especially at the highest shore levels because of their relativeindependence from the substratum

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West lndian Molluscan Communities 379

Where both gravitational stability requirements and the rigors of tem-perature and desiccation are relaxed there is a corresponding relaxation ofmorphological restrictions and an increase in morphological diversity Thisapplies particularly to the cryptic low-shore fauna on sheltered shores and toa lesser extent to the open-surface assemblages on sheltered rocky shoresand the high-level splashpool fauna The rather uniformly small size ofcryptic sheltered-shore species may be related to the small spaces character-istic of the habitat indeed the high-spired shell form (low A2 and W) ofmany species resulting in a relatively very small transverse diameter mayalso be an adaptation to living in limited spaces in crevices and under stonesSmall size in splashpools may be related to osmotic limitations imposed bythe highly variable salinity in this habitat (See Note at end)

The various morphological trends in the different habitats are moreor less independent of the taxonomic composition of the fauna It maytherefore be expected that these trends being general reflections of howthe gastropod body plan interacts with the external physical environmentwill recur in physically similar communities in other parts of the tropicsA preliminary qualitative examination of material collected at intertidallocalities in Brazil West Africa the East Pacific and the Indo-WestPacific lends support to these expectations and strongly suggests thatmorphological composition is a relatively predictable feature of intertidalmolluscan communities under given physicochemical conditions

Despite this predictability of form in physically similar habitats thereare distinct differences between geographically isolated communities aswell as unique features of particular regions The most striking feature ofWest Indian molluscan intertidal communities is the highly diverse faunaof the littoral fringe in particular and the upper shore in general Thefamily Littorinidae is represented on Caribbean shores by about 13 speciesof which eight were encountered at a single locality (Fort Point Loc 1)Of these eight species six occur on open surfaces in the littoral fringeone occurs in littoral-fringe splashpools and one is found in the highintertidal zone No other coast in the world tropical or temperate hassuch a diverse littorinid fauna as that of the Caribbean islands and adjacentcoasts In addition six neritids are known from West Indian rocky shores(Russell 1941) of which five were encountered in this study Four ofthese were encountered at a single locality (Fort Point Loc 1) of whichthree inhabit open surfaces or splashpools in the littoral fringe Whilecertain parts of the Indo-West Pacific (eg Palau and the Philippines)support more species of Nerita than are present in the Caribbean thenumber of West Indian neritid species is greater than in the East Pacific andin West Africa mainland coasts in Brazil appear to lack representativesof the genus Nerita altogether

Another striking feature of the open-surface molluscan assemblage in

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

380 Bulletin of Marine Science [23(2)

the West Indies is the relatively large number of keyhole limpets (genusFissurella) Four species of this genus were encountered in this studybut several others are known from the Caribbean region (Farfante 1943)In the East Pacific only one species (F virescens) appears to be at allcommon on open intertidal surfaces (observations in Panama and south-west Ecuador) Brazilian shores that I have examined also appear tosupport but a single common open-surface species (F elenchi) (Vermeijamp Porter 1971 Vermeij 1972) At Dakar Senegal two species (Fcoarctata and F nubecula) are abundant on open surfaces but only thelatter species could be found in Ghana (Vermeij 1972) Open-surfacespecies of Fissurella appear to be totally absent from most tropical Indo-West Pacific shores (observations in Hawaii Guam Palau Philippinesand Singapore see also Taylor [1968 1971] for observations in the IndianOcean)

The relative profusion of species in such groups as the littorinids neritidsand fissurellids on open surfaces is counterbalanced by the absence ornear absence of certain others which are generally associated with inter-tidal habitats elsewhere in the tropics For example while I have collectedcowries (genus Cypraea) intertidally in West Africa the East Pacific andespecially the Indo-West Pacific I have never observed them either onBrazilian or Caribbean shores despite their presence in these regions atsubtidal levels The pulmonate genus Siphonaria is represented by at leastone limpet species on open intertidal surfaces in Brazil West Africa theIndo-West Pacific and the East Pacific but is absent from all but some ofthe most northern Caribbean shores The genus Conus is also poorlyrepresented relative to Panamic and Indo-West Pacific shores Althoughcertain mitrids do occur under stones on West Indian shores they arenot represented by open-surface species as they are on many Indo-WestPacific shores The Cerithiidae which are highly diversified into crypticand open-surface species in the Indo-West Pacific and to a lesser extentin the Panamic region are represented on Caribbean rocky shores by onlytwo cryptic species and in West Africa by only one cryptic speciesCertain of these differences can perhaps be attributed to the very smalltidal amplitude of the Caribbean as compared to that of most other tropicalshores but others such as the small number of Caribbean and WestAfrican shore cerithiids would appear not to be related to this factor

Apart from taxonomic differences between physically more or lesssimilar communities in geographically isolated regions there would appearto be some more general ecological differences The most important of thesethe high diversity of the littoral-fringe fauna in the Caribbean as comparedto other regions has already been dealt with A second difference wouldappear to be the generally greater size of low-shore open-surface specieson wave-exposed coasts in the Indo-West Pacific and Panamic regions

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 381

as compared to the Caribbean and Pernambuco (see also Vermeij ampPorter 1971)

A third difference between the Caribbean and Indo-West Pacific isrelated to the difference in faunal composition between limestone andvolcanic substrata In the Indo-West Pacific neritids and limpets of theupper shore tend to live either on limestone or lava but not generally onboth while upper-shore littorinids lower-shore neritids and most lower-shore limpets occur on both types of substratum (Vermeij 1971c) Inorder to explain these observations it was argued that the pattern ofspecialization to the substratum is related to the overall adaptive strategiesof the various groups Those with a large area of soft tissue in contactwith the substratum (neritids and limpets) when exposed to the air oftenspecialize to a particular type of substratum especially at high shorelevels where the differences in physical properties of the substrata are mostclearly brought out while those in which the area of contact betweenthe animal and the substratum is small do not Morphological data andobservations on environmental and tissue temperatures (Vermeij 1971b)suggest close adaptation of upper-shore species to the particular physicalregime experienced by them Those few upper-shore neritids and limpetsthat were found on both limestone and lava are known either to be extremelyeurytopic or to have a very limited geographic distribution on small islandsIn the Caribbean however no upper-shore species appears to be specializedto either limestone or volcanic substratum except possibly the littorinidEchininlls nodulosus on limestone and the undescribed Acmaea on sand-scoured lava at Loc 5 On volcanic shores at St Maarten Saba andGuadeloupe the individuals of neritid and littorinid species are muchsmaller than those of the same species on similarly wave-exposed lime-stone shores Although no measures of population density were madeit was very evident that a given area of limestone substratum supportsmany more individuals than comparably rough volcanic surfaces Apossible reason for the lack of substratum specialization among WestIndian upper-shore gastropods may be related to the relative rarity ofvolcanic shores in comparison to the ubiquitous reef-limestone formationsbut available information is too scanty to permit adequate appraisal of thissuggestion

Explanation of the taxonomic and more general differences betweencommunities in different regions under similar physical regimes must awaitfurther documentation of their morphological composition and ecologicalstructure which is now in progress At present however it would seemthat the most substantial differences between physically similar communitiesoccur among open-surface assemblages at low shore levels and particularlyamong communities of the littoral fringe It is intriguing that the geographicdistribution of species in these habitats is significantly more restricted

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

382 Bulletin of Marine Science [23(2)

than in more sheltered open-surface and cryptic habitats Not only maythis reflect steeper horizontal environmental gradients and a greater regionaluniqueness of open-surface wave-exposed habitats (Vermeij 1972) butthe role of historical accident in determining morphological and ecologicalstructure of assemblages may be greater in these more unique com-munities than in the morphologically more diverse cryptic habitats Itis hoped to consider this problem in greater detail in subsequent papers

ACKNOWLEDGMENTS

This study was supported in part by Grant No FA-31777 from theNational Science Foundation and by a Grant in Aid from the Society ofSigma Xi I wish to thank E Zipser J W Porter H ten Hove SOnneweer F Creutzberg and K Bailey-Meyer among others for theirassistance in the field work for this study and L Davies for his assistancein carrying out certain of the geometrical measurements I am also greatlyindebted to the late Professor T F Gareau and to Dr F Creutzberg forthe use of laboratory facilities at the Discovery Bay Marine Laboratory andCaraibisch Marien-Biologisch Instituut respectively

SUMARIO

COMUNIDADES DE MOLUSCOS DE LAS ANTILLAS EN LA ZONALITORAL ROCOSA UN ENFOQUE MORFOLOGICO

Se llevaron a cabo analisis morfol6gicos y biogeograficos de los moluscosgaster6podos de la zona litoral rocosa en cinco localidades de JamaicaCuraltao y Guadalupe en las Antillas Los gaster6podos que viven ensuperficies descubiertas a niveles de costa baja en areas expuestas a lasolas tienen mas bien una uniformemente alta proporci6n de expansionW grandes semiangulos apicales A12 grandes angulos de estabilidadgravitacional posterior 07 y grandes angulos de estabilidad gravitacionallateral Ow las aberturas tienden a ser de ovales a redondas Pueden variargrandemente en tamaiio pero muchas especies tienden a ser relativamentegrandes Las especies de superficie abierta cuando estan en habitats decosta baja mas protegidos pueden tener valores mas bajos de A12 WOT Y Ow que sus iguales mas expuestas a las olas y la abertura puedevolverse elongada Especies del borde de la costa en superficies abiertaspueden tener tambien valores de A12 W OT y Ow algo mas bajos quelos de las especies de costa mas baja expuesta a las olas pero la aberturaes uniformemente de oval a redonda Las especies en esta categoria oscilanun poco menos ampliamente en tamaiio que a nive1es de costa mas bajaLas pocetas de las costas altas contienen una fauna limitada de pequeiiasespecies con aberturas de ovales a redondas y con los valores de A12W 07 Y Ow oscilando ampliamente

Entre las especies de costa baja que viven ocultas en grietas 0 bajo las

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

REFERENCESABBOTT R T

1954 Review of the Atlantic periwinkles Nodilittorina Echininus andTectarius Proc US natn Mus 103 449-464

BATZLI G O1969 Distribution of biomass in rocky intertidal communities on the

Pacific coast of the United States J Anim Ecol 38 531-546BEQUAERT J C

1943 The genus Littorina in the western Atlantic Johnsonia 1 (7) 1-28BORKOWSKI T V AND M R BORKOWSKI

1969 The Littorina ziczac species complex Veliger 11 408-414CODY M L

1968 On the methods of resource division in grassland bird communitiesAm Nat 102 107-147

1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

1la Effects of competition predation by Thais lapillus and other factorson natural populations of the barnacle Balanus balanoides EcolMonogr 31 61-104

1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

1973] Vermeij West Indian Molluscan Communities 383

piedras hay una gran diversidad en todos los parametros excepto en eltamafio que tiende a ser mas pequefio As donde la diversidad deltamafio es mayor la diversidad morfol6gica parece ser menor y vice versaLa relativa uniformidad morfol6gica en las superficies descubiertas de costabaja expuesta a las olas puede reflejar restricciones morfol6gicas impuestaspor la necesidad de adaptarse a la turbulencia del agua Los rigores de latemperatura y la desecaci6n imponen ciertas restricciones en la formaen especies de superficie descubierta del borde de la costa

Aspectos unicos de las comunidades de moluscos litoraIes de las Antillascomparados con los de otras partes de los tr6picos incluyen una fauna delborde de la costa aItamente diferente particularmente de litorinidos unagran diversidad de lepas en superficie descubierta (genero Fissurella) laausencia 0 casi ausencia de grupos litoraIes tipicos tales como los delgenero Cypraea lepas pulmonadas (genero Siphonaria) c6nidos ceritidosy mitridos de superficies descubiertas y una falta de especializaci6n encuanto al substrato entre los neritidos y lepas de costa alta Las diferenciasmas sustanciales entre comunidades litorales fisicamente similares endiferentes partes del tr6pico ocune entre agrupaciones de superficiedescubierta a niveles de costa baja y particularmente en el borde litoralLa distribuci6n geografica de especies en estos habitats es significativamentemen or que en habitats de superficie descubierta mas protegidos 0escondidos

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1970 Chilean bird distribution Ecology 51 455-464CONNELL J H

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1961b The influence of interspecific competition and other factors on thedistribution of the barnacle Chthamalus stellatus Ecology 42710-723

1970 A predator-prey system in the marine intertidal region 1 Balanusglandula and several predatory species of Thais Ecol Monogr40 49-78

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NOTE ADDED IN GALLEYS

While morphological diversity is high in rocky-shore molluscan assemblagesby no means all morphological types found among gastropods are represented inthis habitat In particular such sand- and mud-dwelling gastropods as Turritellaand Terebra have higher spires (lower A2 values) than any rocky-shore spe-cies No planispirally coiled forms (t = 0) occur on rocky shores in spite oftheir very common occurrence in Paleozoic marine assemblages and amongRecent fresh-water molluscs Sinistral coiling which is typical of many fresh-water and terrestrial gastropods as well as of some soft-bottom marine species isnot characteristic of any rocky-shore species known to me although it may occuras a rarity within some normally dextral species The absence of planispiral andsinistral forms among rocky-shore marine molluscs must for the present remainunexplained

386 Bulletin of Marine Science [23(2)

luscan) around Mahe Seychelles Trans R Soc Lond Ser B254 130-206

1971 Intertidal zonation at Aldabra Atoll Trans R Soc Lond Ser B260 173-213

THORSON G1957 Bottom communities (sublittoral or shallow shelf) Mem geol Soc

Am 67 461-538VERMEIJ G J

1971a Gastropod evolution and morphological diversity in relation to shellgeometry J Zoo 163 15-23

1971b Temperature relationships of some tropical Pacific intertidal gas-tropods Mar Bio 10 308-314

1971c Substratum relationships of some tropical Pacific intertidal gastro-pods Mar Bio 10 315-320

1971d The geometry of shell sculpture Forma et Functio 5 319-3251972 Endemism and environment Some shore molluscs of the tropical

Atlantic Am Nat 106 89-101In press Morphological patterns in high intertidal gastropods Mar Bio

20 319-347VERMEIJ G J AND J W PORTER

1971 Some characteristics of the dominant intertidal molluscs from rockyshores in Pernambuco Brazil Bull Mar Sci 21 440-454

WARD J1967 Distribution and growth of the keyhole limpet Fissurella barbadensis

Gmelin Bull Mar Sci 17 299-318WARMKE G L AND R T ABBOTT

1962 Caribbean Seashells Livingston Publishing Company NarberthPennsylvania 348 pp

WOODRING W P1966 The Panama land bridge as a sea barrier Proc Am phil Soc

110 425-433WORK R C

1969 Systematics ecology and distribution of the mollusks of Los RoquesVenezuela Bull Mar Sci 19 614-711

NOTE ADDED IN GALLEYS

While morphological diversity is high in rocky-shore molluscan assemblagesby no means all morphological types found among gastropods are represented inthis habitat In particular such sand- and mud-dwelling gastropods as Turritellaand Terebra have higher spires (lower A2 values) than any rocky-shore spe-cies No planispirally coiled forms (t = 0) occur on rocky shores in spite oftheir very common occurrence in Paleozoic marine assemblages and amongRecent fresh-water molluscs Sinistral coiling which is typical of many fresh-water and terrestrial gastropods as well as of some soft-bottom marine species isnot characteristic of any rocky-shore species known to me although it may occuras a rarity within some normally dextral species The absence of planispiral andsinistral forms among rocky-shore marine molluscs must for the present remainunexplained